OSX: Move to cocoa API (bgo#677866)

This commit moves from the ige-/gtk-mac-integration API to the
GtkOSXApplication API (which is still provided by the same library
called 'gtk-mac-integration', not to be confused with the API name).
This means banshee no longer uses any OS X carbon functions (which are
deprecated), but cocoa only. The Banshee.Osx backend also now fully
relies on gtk-mac-integration as interface to OS X specific functions
and does no longer contain own DllImports into OS X core framework.

This drops the support for closing the banshee main window (via
Cmd+Close) since GtkOSXApplication does not handle this properly right
now. Banshee is a single-window only application at the moment, so this
shouldn't be a usecase anyway.

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>