add sample for multithreading
with Application.Invoke and Application.UITaskScheduler
add an XwtSynchronizationContext class
* uses Application.Invoke to synchronize with UIThread
* install/uninstall context automatically
add ISynchronizeInvoke implementation to XwtComponent
add more multithreading samples
implement ISynchronizeInvoke explicit
Merge branch 'add-multithreading-patterns'

Conflicts:
	TestApps/Samples/Samples.csproj
	Xwt/Xwt.csproj