WPF: Rebuilt XAML style for menus from the ground up.

- Used the standard XAML style as a starting point to fix a few bugs
like
the popup menu wrong placement and making a difference between top level
menu items and submenus. The context menus are still a little buggy
though.
- Added InputGestureText for a few menu items but the shortcuts do not
work yet.
- The loop text box is only selectable when the row is currently
selected.