Bug 727762 - Undo- and redobutton in Undo History Dialog (gimpundoeditor)...

...don't work

Make action_data_get_display() return the right display also if the
action callback's user_data is a GimpImageEditor (GimpUndoEditor in
this case). The undo and redo actions now require a display, not only
an image, so this broke as a side effect.
app: don't create a fallback crosshair in gimp_brush_tool_create_outline()

Instead, draw the crosshair in gimp_brush_tool_draw() if create_outline()
didn't return an item *and* if there is no brush tool cursor.

In gimp_source_tool_draw(), don't add an additional crosshair if
create_outline() returned an item.

This fixes both "no cursor at all" and "both outline and crosshair
shown" for certain prefs settings conbinations (bug #623734).
app: add gimp_paint_tool_set_draw_circle()

which enables drawing a circle with a given radius at the cursor
location.
app: use gimp_paint_tool_set_draw_circle() in the ink tool

so we get at least a rough preview of where the paint will go.