Use text instead of icons for todo scanning scope buttons.

Those icons were initially introduced as stubs and are not
so much informative.

Change-Id: Ib990ef36f41e1094d024c91d2f873728a971d1fa
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>