app: fix and improve text style overlay keyboard interaction
- app/widgets/gimptextstyleeditor.c: don't block all key events so they can reach the canvas. - app/display/gimpdisplayshell-callbacks.c: instead, ignore canvas key events when the canvas has no focus. This fixes navigating out of the text style editor with TAB, moving focus to the canvas. I have no clue why navigating *between* the widgets in the style editor doesn't work.