shortcut keys and right-click menu

if event.key=='left' and event.modifier=='alt':

That isn't the right approach, actually, because it doesn't support
multiple modifiers. I think the better approach is to add attributes
alt, ctrl and shift which are booleans, eg one would do

  if event.alt and event.key=='x' :


  if event.alt and event.ctrl and event.key=='x' :