Change gimp_tool_set_active_modifier_state() to honor the new
GimpToolControlSetting. Explicitly set the mode to SEPARATE in
all tools that require modifier keys during a stroke.
And here comes the actual fix: change GimpTransformTool and
GimpToolTransformGrid to use SAME mode, and remove their
active_modifer_key() and hover_modifier() impls, so it makes no
difference whether a modifier is pressed before of after mouse button
press/release.
and update the grid as soon as a constraint is changed, not only on
the next motion. Change GimpTransformTool to forward the events to the
widget if it exists, but still handle them if it doesn't (yes this
code duplication is ugly, but the widget can hardly handle events if
it doesn't exist...).
Rename gimp_tool_widget_snap_offsets() to set_snap_offsets(),
and add gimp_tool_widget_get_snap_offsets().
Also rename gimp_tool_widget_status() to set_status(), and
add new function and signal set_status_coords().
Simply use g_object_bind_property() to connect the grid properties of
GimpTransformOoptions and GimpToolTransformGrid and remove all other
grid property setting code.
