[Lazarus] OS theme change via TAppProperties

[Alexey Tor.]

This forum topic is about how we can react to macOS theme change.
https://forum.lazarus.freepascal.org/index.php?topic=43111.msg376056;topicseen#new

and I had the idea to make all this task simpler.
we have the TApplicationProperties. let's add OnThemeChange there!
some widgetset funcs will be needed.
now it's the problem - do this reaction on macOS, on win32,... no one 
knows how to do it on gtk2/qt...
I wrote to Dmitry (Cocoa developer) about this idea, then wrote here.

Alexey Torgashin