Fix(Dropdown, TetherContent): add touchstart event to TetherContent, …

[Arsolio]

Just added touchstart event to fix issue with toggle for iOS devices.

Closes #458

[eddywashere]

Is it ever possible to trigger both events or is it always one or the other? I've seen reference to using cursor: pointer as a hack around this issue. That could be a work around for others until I have time to test this.

[haobo-chen]

@eddywashere Hi! I also looked into this issue for sometime and here are some notes you might find useful:

• iOS browser(Safari, WKWebView) will only trigger the touchstart event.
• Desktop browsers will only trigger the click event.
• For Android browsers will trigger click if touchstart is not listened. If both are listened, only the touchstart will be triggered so there is no double triggering issue.

I haven't tested old Android versions but the statement above is true on 6.x and 7.x.

In conclusion, I think this PR is legit and I will be happy to see it get merged. (Currently I'm using the cursor: pointer hack)

[TheSharpieOne]

The changes look good to me. I cannot comment on whether or not the address the issue as I don't have an iOS device.