- 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.