Handle Return in switchpanel.
Explicitly match a Return keypress in the switchpanel, so that the
currently selected window is focused without the keypress event being
handled elsewhere.
Pressing Return to commit the window selection is intuitive but other
shortcuts may bind Return. Any keypress not explicitly handled in the
switchpanel code will be passed through to other shortcut processing.
If, for example, alt+return were mapped to Select Window and the
switchpanel is open, pressing return while holding alt would both commit
the window change AND select the window, which is not intuitive.