Restrict select to left click

Selecting something in a menu (no matter what it contains) should only be possible with a basic left mouse click. Not middle click. Not right click. Not Ctrl+click and so on.

The line is copied from core/resources/src/jquery/jquery.suggestions.js, which means it should be well tested and cover all cases.

This does not solve all problems (the menu still closes because of focus loss) but is a base for more.
It would be nice if this condition could be named.

    if ( isLeftMousebuttonClick() ) {
        // stuff
    }

Not sure how to best go about this in such code though. @adrianlang ping