Add middle-click emulation options for mouse and touchpad

Submitted by Yan Pas

Assigned to Nick Schermer

Link to original bug (#12278)

Description

All touchpads and many mice do not have middle click. It would be fine, if XFCE provided

• 3-finger tap for middle click for touchpads (if touchpad supports multitouch)
• Left+Right buttons click for middle click (for mice and touchpads)

Version: 4.12.0