Pan gesuture not working for Side Under mode

[saja02df]

There is a small issue w.r.t to the support of UIPanGestureRecognizer for mode underCenterPanelLeft.
The side under mode is missing support for the Pan gesture but is enabled programmatically after a swipe-edge gesture.

Looks like we just need to add the following line in SideMenuController+SideUnder.swift::configureGestureRecognizersForPositionUnder:

centerPanel.addGestureRecognizer(panGesture)

Note: The above change is already done for sideOver mode