Fix rich text being pasted by Ctrl+V in plain text embedded composers
This restores normal key handling MainWindow:key_press_event when a keyboard modifier (Ctrl, Shift, etc) is down, keeping the ordering hack only for plain key presses, so that we can handle Ctrl+V before WebKitGTK does, and hence letting us paste plain text default when rich text is not enabled.
Fixes Bug 730495.