[Merge] lp:~nick-dedekind/qtubuntu/1493530-mimeData into lp:qtubuntu

[Nick Dedekind]

> When it gets a null mimeData it should also clear the global (D-Bus)
> clipboard.

I'm not so sure we should be clearing it if it's null. If a client tries to copy an item which isn't able to set mime data, we shouldn't clear the last item in the clipboard.

I believe the last valid clipboard item should persist until another valid item is set. We don't ever "empty" the clipboard as I understand. For this reason we should actually also put the change inside the if block as well.
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https://code.launchpad.net/~nick-dedekind/qtubuntu/1493530-mimeData/+merge/272630
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