Three finger tap middle click does not work on Ubuntu 15.04

[fuelrod]

In 14.10 a three finger tap on a touch-pad was recognised as a middle click. In 15.04 this is no longer the case. It appears to get swallowed up waiting for the various three finger gestures listed here: https://wiki.ubuntu.com/Multitouch

[fuelrod (danielhodder)]

Three finger swipe left and right which used to be back and forwards also appears to no longer work.

[fuelrod (danielhodder)]

Removing (commenting) the following section in plugins/unityshell/src/unityshell.cpp also does not restore the gestures and taps (even though it removed the unity bindings to the three finger taps)

4025 gestures_sub_windows_.reset(new nux::GesturesSubscription);
4026 gestures_sub_windows_->SetGestureClasses(nux::TOUCH_GESTURE
4027 | nux::DRAG_GESTURE
4028 | nux::PINCH_GESTURE);
4029 gestures_sub_windows_->SetNumTouches(3);
4030 gestures_sub_windows_->SetWindowId(GDK_ROOT_WINDOW());
4031 gestures_sub_windows_->Activate();