Update ReliefFrame and fix gradient surface leaks
Fix image surface memory leak
Improve disposal of resources

Backends can now opt-in to automatic disposal of resources in the
UI thread.
Dispose tookit backends when disposing the main App
[Gtk] Dispose Cairo resources in the UI thread
Fix some ScrollView issues

Allocate the ScrollView child after the child size has been set. This is
necessary sine Reallocate relies on the actual child size to allocate
its own children.

Also fixed allocation issue in the mac backend.
Added some logging to help debugging allocation bugs
Fix SizeConstraint.ToString
[Gtk] Fix leaked ImageSurface
Throw a more meaningful exception when starting a drag without a drag image set

Previously, it threw an NRE, which doesn't tell you what's actually wrong.
Add Desktop.MouseLocation API with Gtk, Mac and WPF implementations
[WPF] Fix build
Remove ReliefFrame
[Mac] Vertically align labels and single-line text entries
[Mac] Fix NRE when adding a widget to a dialog
Properly initialize scroll adjustments for widgets using custom scrolling
[GTK] Use the new api to enable support for retina icons
Merge branch 'master' of git://github.com/mono/xwt into xwt_upgrade