x/mobile/event/mouse: implement helper method for modifier keys

[robpike]

It is conventional that on a laptop with only a trackpad button, option and command (on Macs) turn a click into a middle or right click. It would be nice to have this functionality provided once, in the mouse package.

[changkun]

Is there anything we can really do here? The current mouse.Event already convey the Button and Modifiers information. This is already enough for users who would, or would not, like to have the described conversion.