Depends on D20085
I guess this brings us back to the old debate of a generic solution vs. patching all the UIs. I'm inclined to agree with this approach because it should probably be up to the specific UI what it wants to happen on the gesture.
It means we need to be careful how we use it, though. On the desktop we also have press and hold to engage applet move mode, which means it's already off-limits for applet context menus there. Is this change going to conflict with that wrt/ user expectations?