fix mouse event listener change before term attached

[pfitzseb]

Whenever a terminal would receive a \x1b[?1003h or \x1b[?1003l escape code (e.g. when opening a tmux session) before being attached with terminal.open(element) the following internal error would be thrown:

Uncaught TypeError: Cannot read property 'classList' of undefined
    at InputHandler.resetMode (InputHandler.ts:1482)
    at Array.<anonymous> (InputHandler.ts:161)
    at EscapeSequenceParser.parse (EscapeSequenceParser.ts:507)
    at InputHandler.parse (InputHandler.ts:299)
    at Terminal._innerWrite (Terminal.ts:1382)
    at Terminal.ts:1351

This PR fixes that issue and should still apply the correct classes at all times.

[Tyriar]

Nice catch, I guess the element will not have the class if open after entering tmux though? Could you also add this in open too?

if (this.mouseEvents) {
  // add class..
}

[pfitzseb]

I did. The second branch there should technically be pointless but can't hurt to have I think.

[Tyriar]

@pfitzseb doh how did I miss that 😅