[MonoDevelop] Added XamMac templates conditions

These conditions make the Xamarin.Mac templates not showing up
in Xamarin Studio when Xamarin.Mac is not installed.

- Added MonoMac file templates and project templates condition.
- Added Xamarin.Mac file templates and project templates condition.
Merge pull request #888 from VincentDondain/master

[MonoDevelop] Added XamMac templates conditions