gestures: Restrict actions based on keybindingMode

Just like keybindings and the message tray pointer barrier, gestures
don't always make sense - for instance, swiping up the screen shield
should not trigger the message tray just as the SelectArea action around
the left edge should not open the overview.
To avoid this, restrict gestures based on the current keybinding mode.

https://bugzilla.gnome.org/show_bug.cgi?id=740237
Rename KeyBindingMode to ActionMode

The keybinding mode is no longer used exclusively for actions triggered
by keybindings, so reflect this by a more generic name.

https://bugzilla.gnome.org/show_bug.cgi?id=740237
windowManager: Update stacking during workspace switches

When animating workspace switches, windows on the old and new workspaces
are temporarily reparented. If windows are restacked, those windows will
thus be ignored by mutter until meta_switch_workspace_completed() resyncs
the stacking at the end of the animation.
As a result, activating a window on another workspace that is not on top
of the stack is very noticeably a two-step operation of switching workspace
and raising the window. There is a technical reason for that order[0], but
we can avoid the visible disruption by manually syncing the stack during
the switch operation.

[0] https://git.gnome.org/browse/mutter/tree/src/core/workspace.c#n590

https://bugzilla.gnome.org/show_bug.cgi?id=741680
Bump version to 3.15.3

Update NEWS.