Add ontouchstart handler for checkbox.

[patleeman]

This commit adds a ontouchstart handler to the checkbox format to allow checking and unchecking on a mobile device while focused on editor.

Based on comment by @sferoze on this issue.

[jhchen]

Thanks!

[sferoze]

@jhchen @patleeman

There is a mistake in this commit.

On mobile devices register both click and touch start events. So if the editor is not focused, and you check a box, it checks and then unchecks as both event handlers are called.

You only want to add one or the other.

Add touchstart handler if ('ontouchstart' in window)

otherwise, add the normal click event.

[jhchen]

Good catch @sferoze! I did not notice the unfocused case when merging. Can you submit a PR?

[sferoze]

@jhchen I just submitted a pull request with this specific fix

sferoze wants to merge 1 commit into quilljs:develop from sferoze:quill-checkbox-touchstart-fix