Remove incomplete ModalWindow implementation for release
Fix compilation error
Add DateTimePicker tests, and fix issues. Fixes #87
WinForms: Setting value outside of Min/Max date bounds should not throw an exception, but rather just limit to date range
Gtk: Fix sizing of DateTimePicker on some platforms, and restrict date instead of showing that it is invalid (red)
Mac: Re-enable clearing of the date with Cmd+Delete key
Remove incomplete DrawableCell for release
Wpf: Update Extended Wpf Toolkit to use newer DateTimePicker. Almost works, so still need to work around clip issues with the mouse.
Mac: Fix setting DateTimePicker.Value to null