Tidying mouse and touch event handling

Fixes issues caused by Photoswipe jumping to the conclusion that mouse or touch can be deactivated depending on what is used first. This conclusion is not good on touch enabled laptops using browsers which only generate touch events when touching and mouse events when using the mouse, like Chrome and Opera 15+ on Win 8.
Also fixes an error in the deactivation of the touchstart handler in onMouseDown, which can ultimately disable both mouse and touch in Chrome and Opera 15+.
Changes successfully tested on the latest version of major browsers on Windows, Android, iOS and OS X.