Add Exit option to the context menu

[SumatraPeter]

There seems to be no way to close the viewer if one is in fullscreen mode with the toolbar turned off.

An easy solution would be to add an Exit option to the right-click context menu (especially in fullscreen mode, but might be useful for mouse users otherwise too).

(Note that this is not the same as the prior feature request made to add a Close button to the toolbar itself.)