macOS: Add AppBundle support

[Ghabry]

The CMakeLists lacks App bundle support.
Also there is no suitable icon and no suitable plist file.

@tyrone-sudeium can you help? :)

For "macqtdeploy" see https://stackoverflow.com/questions/51556088/cmake-copy-dependencies-to-executable-output-path/51569345 (tested it, works)

[tyrone-sudeium]

Will check it out this weekend if I get time :)

[Ghabry]

Oh and please set the "Retina support" flag. High DPI support is essential (though I will use Windows when fixing DPI issues, shouldn't make a huge difference because it is Qt ;))