app: add initial version of GimpDial, a widget to select a range of angles

Ported from the color-rotate plug-in.
Bug 723534 - Warp transform tool does not follow active layer

Add GIMP_DIRTY_ACTIVE_DRAWABLE to the warp tool's dirty mask, so it
correctly switches to the new active layer. Also remove
GIMP_DIRTY_IMAGE_STRUCTURE, I don't know why that was there in the
first place.