frontend/cmake: Enable Qt strict mode

Qt strict mode disables APIs deemed "'suboptimal' or 'dangerous'" [1]
and "clearly undesirable" [2] by Qt that will be removed in the long
term.
Usages of the APIs in OBS have been removed in the previous commits, and
by setting this flag we keep ourselves from using them again.
The versioning works in a way where e.g. a new addition to this in 6.9
would only be disabled if at least 0x060900 is set. By setting 0xFF0000,
we're effectively disabling any APIs that are deemed to be bad in any
future Qt version (up to 255).
While this could lead to OBS not being buildable against bleeding edge
Qt, it also means that it will be noticed early. Should the disabled API
turn out to be too complex to remove, the value can be downgraded to the
last working one then.

[1] qt/qtbase@3a6c8e0
[2] qt/qtbase@f9163ae

Author: gxalpha

frontend/cmake/ui-qt.cmake

@@ -18,6 +18,9 @@ set_target_properties(
   PROPERTIES AUTOMOC TRUE AUTOUIC TRUE AUTORCC TRUE AUTOGEN_PARALLEL AUTO
 )
 
+# https://doc.qt.io/qt-6/qtglobal.html#QT_ENABLE_STRICT_MODE_UP_TO
+target_compile_definitions(obs-studio PRIVATE QT_ENABLE_STRICT_MODE_UP_TO=0xFF0000)
+
 set_property(TARGET obs-studio APPEND PROPERTY AUTOUIC_SEARCH_PATHS forms forms/source-toolbar)
 
 target_sources(