qt_add_plugin should accept sources argument

# https://github.com/qt/qtwayland/blob/dev/examples/wayland/custom-shell/client-plugin/CMakeLists.txt

qt_add_plugin(exampleshellplugin)

target_sources(exampleshellplugin PRIVATE
    main.cpp
    exampleshellintegration.cpp exampleshellintegration.h
    examplesurface.cpp examplesurface.h
)

qt_add_plugin(target
              [SHARED | STATIC]
              [CLASS_NAME class_name]
              [OUTPUT_TARGETS variable_name]
)

qt_add_library(target
    [STATIC | SHARED | MODULE | INTERFACE | OBJECT]
    [MANUAL_FINALIZATION]
    sources...
)

qt_add_executable(target
                  [WIN32] [MACOSX_BUNDLE]
                  [MANUAL_FINALIZATION]
                  sources...)

add_library(<name> [STATIC | SHARED | MODULE]
            [EXCLUDE_FROM_ALL]
            [<source>...])

add_executable(<name> [WIN32] [MACOSX_BUNDLE]
               [EXCLUDE_FROM_ALL]
               [source1] [source2 ...])