Clang: be more explicit about marking third-party code.

- This is only a partial solution to warnings caused by third-party
   code, since #including the headers from other sources won't apply
   the warning-suppressions.
 - Flags are not applied when building the source as part of a larger
   target, but are on re-building just one object (it seems -- CMake
   issue to track down).

src/libcalamares/CMakeLists.txt

@@ -32,8 +32,7 @@ set( kdsagSources
     kdsingleapplicationguard/kdtoolsglobal.cpp
     kdsingleapplicationguard/kdlockedsharedmemorypointer.cpp
 )
-set_source_files_properties( ${kdsagSources}
-    PROPERTIES COMPILE_FLAGS "${SUPPRESS_3RDPARTY_WARNINGS}" )
+mark_thirdparty_code( ${kdsagSources} )
 
 include_directories(
     ${CMAKE_CURRENT_BINARY_DIR}

src/libcalamaresui/CMakeLists.txt

@@ -1,6 +1,6 @@
 project( libcalamaresui CXX )
 
-list( APPEND calamaresui_SOURCES
+set( calamaresui_SOURCES
     modulesystem/CppJobModule.cpp
     modulesystem/Module.cpp
     modulesystem/ModuleManager.cpp
@@ -29,7 +29,15 @@ list( APPEND calamaresui_SOURCES
     ViewManager.cpp
 )
 
-list( APPEND calamaresui_UI
+# Don't warn about third-party sources
+mark_thirdparty_code(
+    utils/ImageRegistry.cpp
+    utils/qjsonitem.cpp
+    utils/qjsonmodel.cpp
+    widgets/waitingspinnerwidget.cpp
+)
+
+set( calamaresui_UI
     utils/DebugWindow.ui
 )