Bug 1059650 - [Browser]Back button stops working after clicking '+' icon multiple times

* Change 'mousedown' event to 'touchstart' event for tabsSwipeMngr to fix 'this.currentTab is null' problem.
* Stop screenSwipeMngr gestureDetector right after new tab button clicked.
Merge pull request #26685 from begeeben/1059650_back_button_stops

Bug 1059650 - [Browser]Back button stops working after clicking '+' icon multiple times, r=bfrancis & dale