[Xwt.Mac] Make this csproj compilable via XBuild

First I made this a regular .NET library project and not a monomac
project. I then removed the import for the non-existent monomac
targets file. Finally I added the 'ReferencePath' attribute so that
if we cannot find 'monomac' in the Gac we will then look for it
in a parallel checkout of monomac.

With this changes Xwt.Mac still compiles as normal in MonoDevelop
but when compiled from the commandline as part of MonoDevelops build
we can now succesfully compile this and have it as part of the sln
[RichTextView] Add a plain text format
[WPF] Fix layout issue when replacing the root widget of a window

When replacing the root child of a window, the content was not
properly reallocated.
LisBox implementations

Reuse the IListViewEventSink for the ListBox widget.
Initial implementation of Mac ListBox
[Mac] Improvements to FrameBackend and ExpanderBackend
[Mac] Implement DispatchPendingEvents

And fix some unit test issues
[Mac] Fix ImageViewBackend sizing issue
[Mac] Don't assume XWT controls the entire view hierarchy

We were getting an NRE when we reached a superview that's not an IViewObject
Create widget Font object with the same toolkit as the widget
[Mac] CheckBoxBackend: remove default button title
[Mac] Fix screen coordinate calculation

The screen calculation formula has to be tweaked
when the widget is marked as flipped.
All mac tests are green now.
[Mac] Fix MonoMac assembly reference in test app
Improvements in the Context class

The TransformPoints and Distances methods are now
implemented in Context, so it is not necessary to provide
an implementation in the backend.

Improved the Distance struct.
Merge remote-tracking branch 'origin/master' into visual-render
Fixes in the context backend. Added missing files.