Fix pasting images into browser

This patch binds the handlePaste() function to the clipboardData paste event.

Currently firefox/safari do not fully support clipboardData. Therefore
on those browsers the clipboard contents is pasted as plain/text and a
message is displayed within the alert area.

This is a possible solution for #7392

I only tested on:
Firefox (linux) -> NOT working
Chrome (linux) -> working
Firefox (Mac) -> NOT working
Safari (Mac) -> NOT working

Tomorrow I will test on Windows (Firefox, Chrome, IE 10)

For Firefox there is currently some work in progress to get it working too:
https://bugzilla.mozilla.org/show_bug.cgi?id=891247

Before this will be merged I probably have to change something.
Please review and comment. It would be great to get this merged because the pasting of images is a great feature.