Add some more modern syntax in CMake files

[gikari]

Here:

• Config mode syntax for find_package. This syntax is recommended by CMake and official Qt 6 documentation (works for Qt 5 too).
• CMAKE_AUTORCC variable for less verbosity
• add_compile_definitions is recommended by CMake, as it superseeds the add_difinitions.
• Use target_sources instead of variables (more reliable and flexible) for source additions
• Add headers to sources, so that some IDE could pick up them (like Qt Creator).
• Some other small fixes