Add multithreading patterns

I've added some multithreading features including a "Multithreading" sample, which starts different worker patterns and invokes results in the gui thread.

**XwtSynchronizationContext**: is installed automatically and uses Application.Invoke for synchronization. BackgroundWorker can synchronize with the UI Thread automatically for example.

**XwtComponent**: implements the old fashioned ISynchronizeInvoke. Very useful when porting old WinForms Implementations to Xwt.

**Possible regressions**: should be tested in Guest mode

I've tested everything on Linux, Windows (Wpf and Gtk) and Mac (Cocoa and Gtk)

PS: let me know if I should split this in different PRs. The first commit simply adds the sample part without the new implementations and could be very useful for others.