Notify the user when pasting fails

When the user tries to paste some text using Ctrl+V on the desktop (and in the future maybe also on mobile devices) and the content of the clipboard is faulty and thus cannot be pasted completely or at all, the user should be notified of that.

In https://bugs.launchpad.net/ubuntu-calculator-app/+bug/1480662 we came to the conclusion that it would be best to have the input field flashing up. We'd need to ensure that this applies to the whole input field as otherwise it cannot be seen if the input field is empty.