* app/widgets/gimptextproxy.c: also swallow the "toggle-overwrite"
signal.
* app/tools/gimpdrawtool. [ch] (gimp_draw_tool_draw_text_cursor): add
"gboolean overwrite" which causes the cursor to be drawn as block.
* app/tools/gimptexttool.[ch]: implement overwriting, toggling it,
and changing the text cursor accordingly.
