Mouse button modifiers for "v-on" should have more precise aliases

What problem does this feature solve?

Currently mouse button modifiers, introduced in #4866, are left, middle, right for $event.button values 0, 1, 2 correspondingly. But that is only correct for the "default" right-handed layout. In the left-handed mode (when the buttons are switched in the OS) the right button has 0 value and the left button - 2. Besides, also some right-handed users may reverse or remap the mouse buttons. And working with code with these names for mouse buttons may lead to confusion and bugs among developers not familiar with the "default" right-handed mode being the target for these modifiers.

What does the proposed API look like?

Keep left and right as deprecated and add main and secondary as their aliases.

I am currently working on it. What names should the modifiers really have?

1. Since the "mouse" modifiers for left and right have already clashed with keyboard modifiers left and right (v-on:keydown.right and left seems to be broken in Vue 2.2.1 #5046, fix mouse modifier override the keycode ( Fixed #5046 ) #5060), and to avoid such future name issues, should we perhaps include something pointer device / mouse related like ptrMain? Introducing two-word modifiers doesn't seem like a good idea though because of all the case related stuff.
2. What should the main signifiers be?

• for 0 (now left) - main / primary / your suggestions
• for 1 (now middle) - aux (but might be confusing since auxclick is fired for all the mouse buttons except the primary one) / your suggestions
• for 2 (now right) - secondary / your suggestions

Previous names left, right, middle of course are kept but deprecated with warnings.

[dsbert]

If using primary and secondary, then tertiary would be the consistent name for the third, typically middle, button.

The discussion kinda moved to the pull request, and Evan okayed the naming.