Partitura Magica Code2

some experimental dx views

slightly improved the menu

reviewed todo prios and impl. selecting of nearest box
after removing a box

bugfix: play shall not be started if no box exists

clean separation of ClosePartituraCommand and CloseApplicationCommand
bugfix: now using the wpf specific common dialogs instead of windows forms

new extension method CommandExtensions.TryExecute()

refactored UpdateCommand to use new abilities of
Rect class.
Rect class can perform update operations with respect to
MinSize and a "outer rectangle"
tests pass

introduced own immutable Rect class
all tests pass

removed unused assembly refs
before adding own Rect class