Fix alt+selection while mouse events are enabled
Fix alt+selection while mouse events are enabled

Fixes #970

sourcelair/xterm.js,Tyriar/xterm.js,akalipetis/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,akalipetis/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,akalipetis/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,akalipetis/xterm.js,xtermjs/xterm.js