Grease pencil tool now uses an icon for each tool,
previously it mapped icons to tool icons which is now not needed
because for each tool a different brush can be selected which has it's
own icon.
- Tweaked icons for Box Select, Circle Select & Lasso Select.
- Tweaked icons for Rip Region & Rip Edge.
- Added icons for Bone Envelope, Bone Size, Pose Breakdown,
Pose Push & Pose Relax.
- Added icons for curve Extrude, Extrude Cursor.
Also removed grease pencil mirror tool which wasn't an active tool,
and is now consistent with regular Edit Mode.
