OSX: remove dock icon for xpra or make it usable

[Antoine Martin]

Another one of Apple's rubbish "design" decisions means that we can't programmatically disable the dock when we launch an app like xpra...
So we have two options:

• write lots of code to add dock support to xpra
• do lots hackery do have two different plist manifests in the same application bundle

For details, see:

• ​Feature requests versus the "right way" to do it
• ​LSUIElement

[Antoine Martin]

The next build will contain the following enhancements:

• xpra now includes a small chunk of osx MacDock code to make it display the xpra icon
• the python executable is renamed to 'xpra' (see r3321) so osx will show it as "xpra" rather than just "python".

Still TODO: hook into the menu to make it a bit nicer too.
This is now a lower priority.

[Antoine Martin]

already implemented