cmake,UI: Refactor find_qt macro

If QT_VERSION is not defined, it gets define with the AUTO value. And
its definition is moved to the helper file.

find_qt now:
- Check quietly for Qt5 and Qt6
- If QT_VERSION is set to AUTO. It checks firstly if Qt5 was found it
  will use it. If not it do the same for Qt6
- If QT_VERSION is set to 5 or 6, it checks if the choice was found and
  use it. And if not, it falls back to the other if found.
- If neither Qt5 or Qt6 are found, a fatal error is emitted.
- The macro saved the _QT_VERSION in the cache to replace QT_VERSION,
  so the process is not repeated each time that find_qt is used.
- When Qt::Gui is in the Linux component list, Qt::GuiPrivate is added.
  So using the versioned one is no longer required.

UI/frontend-plugins/frontend-tools/CMakeLists.txt

@@ -3,10 +3,7 @@ project(frontend-tools)
 add_library(frontend-tools MODULE)
 add_library(OBS::frontend-tools ALIAS frontend-tools)
 
-find_qt(
-  VERSION ${QT_VERSION}
-  COMPONENTS Widgets
-  COMPONENTS_LINUX Gui)
+find_qt(COMPONENTS Widgets COMPONENTS_LINUX Gui)
 
 set_target_properties(
   frontend-tools
@@ -49,7 +46,7 @@ target_link_libraries(frontend-tools PRIVATE OBS::frontend-api OBS::libobs
                                              Qt::Widgets)
 
 if(OS_POSIX AND NOT OS_MACOS)
-  target_link_libraries(frontend-tools PRIVATE Qt${QT_VERSION}::GuiPrivate)
+  target_link_libraries(frontend-tools PRIVATE Qt::GuiPrivate)
 endif()
 
 if(ENABLE_SCRIPTING AND TARGET OBS::scripting)