[Mac] Auto-copy icon files (and meta?) after building

[GWRon]

For now BMK creates win32 resources automatically during the build process.

• creates meta files
• convert .ico to .rc
• creates resource files ...

For Mac OS we currently seem to require a custom "[sourcefile.]post.bmk" to achieve a similar effect. Maybe it would be good to behave similary on Mac than on Windows.

• if a "sourcefile".ico exists (and not an icns) then create icns
• if a "sourcefile".icns exists, copy it to the resources folder
• if Mac OS allows for meta stuff (or signing?) then handle that too (if there is a corresponding definition file)

For now there is a default icns file used from within BMK but this is not configurable yet.