macos dock icon per forwarded application

[Antoine Martin]

Assuming that this is even possible (TBC), we would need to generate an application bundle on-the-fly for each application we forward. (and ship png2icns to convert icons - even pillow uses exec to save icns files: ​https://github.com/python-pillow/Pillow/blob/master/PIL/IcnsImagePlugin.py)
Then this process would need to send messages back to the xpra client process to tell it to:

• raise windows when the dock icon is clicked
• close those windows when the dock icon's "quit" option is clicked
• this may help with showing native notifications, see ticket:1305#comment:5
• this may help with forwarding application menus too

We should still keep xpra's own dock icon for accessing settings, etc