Add the ability to enable both wheel and drag modes at the same time

[jledentu]

Hi,

Here is a PR thats adds the ability to enable both wheel and drag modes at the same time, as suggested in #239. I propose to split the wheel/drag configurations like this:

zoom: {
      wheel: {
        enabled: true,
        speed: 0.1,
        modifierKey: null
      },
      drag: {
        enabled: true,
        backgroundColor: 'yellow',
        borderColor: 'black',
        borderWidth: 1,
        threshold: 1
      },
      mode: 'xy',
}

speed, threshold, wheelModifierKey (now wheel.modifierKey) are moved to their respective configurations, I think that makes clearer that a given option refers to a given mode.

[kurkle]

Nice!
How about pinch zoom? I think its still checking for zoom.enabled. Maybe it should be possible to disable that separately? Own scope for it?

[jledentu]

Oh you're right, I admit that I often forget to check touch behaviors. Do I add pinch.enabled? I guess that it will often be aligned with wheel.enabled as I don't know if it makes sense to explicitly enable zoom by mouse and disable zoom on touch devices. But at least we allow more flexibility.

[kurkle]

I think pinch.enabled would be good. Maybe fall back from each scope to zoom.enabled, to make the "break" of the change smaller and maybe easier to find.

Edit: Other option would be to add a warning when zoom.enabled is set, about it having no effect. Maybe in the start hook, so it won't be spammy.

[jledentu]

@kurkle I added pinch.enabled + a warning on zoom.enabled as suggested (I think it's better to remove this option in order to keep the code clean and make the configuration more explicit?).

[etimberg]

Two very minor comments. Looks great!