Pull mouse mode handling into its own module

[Tyriar]

The xterm mouse handling code is fairly self contained and could be pulled into its own module:

xterm.js/src/Terminal.ts

Lines 783 to 793 in ddc9466

	/**
	* XTerm mouse events
	* http://invisible-island.net/xterm/ctlseqs/ctlseqs.html#Mouse%20Tracking
	* To better understand these
	* the xterm code is very helpful:
	* Relevant files:
	* button.c, charproc.c, misc.c
	* Relevant functions in xterm/button.c:
	* BtnCode, EmitButtonCode, EditorButton, SendMousePosition
	*/
	public bindMouse(): void {

[Tyriar]

Done by @jerch