... changing layers and warping layer B

Add a new GimpToolControl::dirty_action field, which specifies the
tool action to perform when the a dirty event matching the tool
control's dirty mask occurs; this field defaults to HALT.  Apply
this action to the active tool in tool-manager in response to a
matching dirty event, instead of unconditionally halting the tool.
Likewise, use this action to stop the active tool in response to a
button-press event on a different drawable in the same image.

Set the dirty action of the gradient and warp tools to COMMIT, so
that they get comitted, rather than stopped, in cases such as
switching layers (including switching to/from quick-mask mode),
and, for the warp tool, changing the selection.