[Mac] Use Cocoa for main menu, for great justice!
[Mac] There can only be one (services menu)

Fixes crash when enabling/disabling/adding/removing addins.
[Mac] Re-use root menu else MacOS messes up badly
[MacPlatform] Remove some duplicated NSApplication.Init calls

We need to enforce that NSApplication is only initalised at application
startup. We still call NSApplication.Init more than once and we still
do it lazily which is Bad. We can end up (re)initializing on the wrong
thread and break monomac.