Fix handling pastes in browsers that don't support ClipboardEvent.clipboardData

[javan]

Previously when Trix received a paste event with no clipboardData, we'd do nothing and assume the mutation observer would detect the change and cause an HTML re-parse. But because Trix.MutationObserver doesn't deeply traverse nodes, the mutation summary it returns could be empty depending on the pasted HTML. With a little logging, you can see this by inserting HTML into the editor:

$("trix-editor").insertAdjacentHTML("beforeend", "hello")
=> mutationSummary = {"textAdded":"hello"}

$("trix-editor").insertAdjacentHTML("beforeend", "<p>hello</p>")
=> mutationSummary = {}

Instead of doing nothing, we can capture the paste using getPastedHTMLUsingHiddenElement, which a) fixes the problem of nothing pasting at all, and b) allows Trix to format insert the pasted HTML better.

Pasting in IE11 before:

After: