AddElementPopoverComponent button submits edit form

[GuySartorelli]

The StatelessHoverBar react component omits the button type, which makes it default to being a submit button. This means pressing the enter key while in an input field triggers this button instead of a true form submit button, and also sends unnecessary form submit events when the form isn't actually being submitted.

#869 was intended to fix this, but was closed due to causing another issue - so the problem is still not resolved.