Revert "Remove incomplete DrawableCell for release"

This reverts commit 334b504c04eb5775ca3a58b4400c54910ee63779.
Revert "Remove incomplete ModalWindow implementation for release"

This reverts commit c417c2bff1b2535980a4e2314ce2ecf7dfbf6e28.
Added MenuBar.CreateStandardMenu(), and AutoTrim to simplify creating menus
- Application.CreateStandardMenu() is now obsolete, and by default you no longer need to call menu.Items.Trim().
Remove Eto.CancelEventArgs in favour of using System.ComponentModel.CancelEventArgs now that it is ubiquitous
Fix wpf compile error