Porting rviz plugin to rviz2 (cmake trouble)

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I am trying to port a working rviz1 display plugin (ROS Kinetik) to rviz2 (ROS2 Crystal). I got pretty far, but now I am stuck at creating a shared ".so" library with cmake to be used by rviz2. I end up with either a static ".a" library or errors (which I assume are related to MOC file processing) during compilation. (I am not exactly a cmake wizard, so it may well be easy to solve.)

My pretty simplistic test workspace looks something like this:

src
└── traffic_sign_delegation_manager
    ├── icons [...]
    ├── images [...]
    ├── msg [...]
    ├── ui
    │   ├── traffic_sign_delegation_manager_panel.ui
    │   ├── traffic_sign_delegation_manager.qrc
    │   └── traffic_sign_list_item.ui
    ├── CMakeLists.txt
    ├── package.xml
    ├── plugin_description.xml
    ├── adv_interaction_groupbox.cpp
    ├── adv_interaction_groupbox.h
    ├── draw_area.cpp
    ├── draw_area.h
    ├── traffic_sign_delegation_manager_display.cpp
    ├── traffic_sign_delegation_manager_display.h
    ├── traffic_sign_delegation_manager_panel.cpp
    ├── traffic_sign_delegation_manager_panel.h
    └── ...

I converted my old catkin CMakeLists.txt to the below one, by looking at the migration guide and the available plugins for rviz2. It does compile (via colcon build), but it creates a static .a library - and rviz2 wants a .so library. These are the things I tried to fix it:

• If I add the SHARED keyword to the add_library command like this: add_library(delegator_lib SHARED ...), I get this strange error: /usr/bin/ld: cannot find -lXAW_LIBRARY-NOTFOUND. Interestingly, in the catkin version, the SHARED keyword was not necessary to get a .so library. My feeling is that this is still the way to go... But why doesn't it work?
• If I omit AUTO_MOC and do it the old-fashioned way (by uncommenting the qt5_wrap_cpp macros below), I end up with fatal error: ui_traffic_sign_delegation_manager_panel.h: No such file or directory (and the same for ui_traffic_sign_list_item.h).
• If I activate both AUTO_MOC and the old macro, I get a multiple_instances error for the four MOC files - which makes sense...

At this point, I have no idea what else to try. I'd be very grateful for ANY suggestions!

(I'm on Ubuntu xenial 16.04.)

CMakeLists.txt:

project(traffic_sign_delegation_manager)

set(CMAKE_CXX_STANDARD 17)

if(NOT WIN32)
  add_definitions(-fPIC)
endif()

if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  add_compile_options(-Wall -Wextra -Wpedantic -Wno-deprecated-declarations)
endif()

set(CMAKE_INCLUDE_CURRENT_DIR ON)
set(CMAKE_AUTOMOC ON)

find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(rviz_common REQUIRED)
find_package(std_msgs REQUIRED)
find_package(rosidl_default_generators REQUIRED)
find_package(rosidl_generator_cpp)
find_package(pluginlib REQUIRED)

find_package(Qt5Core REQUIRED)
find_package(Qt5Gui REQUIRED)
find_package(Qt5OpenGL REQUIRED)
find_package(Qt5 REQUIRED COMPONENTS Widgets)

set(msg_files
  "msg/TrafficSignList.msg"
  "msg/TrafficSign.msg"
  "msg/TrafficSignSetList.msg"
  "msg/TrafficSignSet.msg"
  "msg/TrafficSignSetStatus.msg"
  "msg/TrafficSignsManaged.msg"
  "msg/AccLever2.msg"
  "msg/VehicleOdometry.msg"
)

rosidl_generate_interfaces(${PROJECT_NAME}
  ${msg_files}
  DEPENDENCIES std_msgs
)

link_directories(${ament_cmake_LIBRARY_DIRS})

add_definitions(-DQT_NO_KEYWORDS)

qt5_wrap_ui(QT_UI_FILES ui/traffic_sign_delegation_manager_panel.ui)
qt5_wrap_ui(QT_UI_FILES ui/traffic_sign_list_item.ui)

#qt5_wrap_cpp(MOC_FILES
#  traffic_sign_delegation_manager_panel.h
#  traffic_sign_delegation_manager_display.h
#  adv_interaction_groupbox.h
#  draw_area.h
#)

qt5_add_resources(QT_QRC_FILES ui/traffic_sign_delegation_manager.qrc)

set_property(SOURCE traffic_sign_delegation_manager_panel.h PROPERTY SKIP_AUTOMOC ON)
set_property(SOURCE draw_area.h PROPERTY SKIP_AUTOMOC ON)
set_property(SOURCE adv_interaction_groupbox.h PROPERTY SKIP_AUTOMOC ON)
set_property(SOURCE traffic_sign_delegation_manager_display.h PROPERTY SKIP_AUTOMOC ON)

add_library(delegator_lib      # SHARED
  vec2d.cpp
  vec2d.h
  traffic_sign_delegation_manager_panel.cpp
  traffic_sign_delegation_manager_panel.h
  draw_area.cpp
  draw_area.h
  traffic_sign_delegation_manager_display.cpp
  traffic_sign_delegation_manager_display.h
  adv_interaction_groupbox.cpp
  adv_interaction_groupbox.h
  ui/traffic_sign_list_item.ui
  ui/traffic_sign_delegation_manager_panel.ui
  ${QT_UI_FILES}
  ${MOC_FILES}
)

rosidl_target_interfaces(delegator_lib ${PROJECT_NAME} "rosidl_typesupport_cpp")

target_include_directories ...