support pan by middle-click
support pan by middle-click

This has been requested in ticket #115.

yuhangwang/pv,yuhangwang/pv,yuhangwang/pv,biasmv/pv,biasmv/pv,CyrusBiotechnology/pv,CyrusBiotechnology/pv,yuhangwang/pv,biasmv/pv,CyrusBiotechnology/pv,biasmv/pv,CyrusBiotechnology/pv