Create context menu as a clone of the Actions menu. This is a bit
easier than it was in WinForms because the ICommand stuff provides
common routing and enable/disable logic for all instances. (It's
one of the few places where WPF has made my life easier. Yay WPF.)
Added CanExecute tests to existing items. Currently they just
check to see if the project is open.
Wired up File > Close.
fdbd5b89e9