Drop selection when the active buffer changes
Drop selection when the active buffer changes

Fixes #732

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