Only access the clipboard when we really need it
- remove two superfluous sig/slot connections in KisClipboard. These QClipboard modes are not relevant for Krita, and were fired together with dataChanged, resulting in clipboardChanged being called three times.
- remove the preview() method which was only used by the custom image widget
- set the default w/h for the imagefromclipboard widget to 1, so we don't need to retrieve the QImage from the clipboard just to get the actual dimensions
- simplify KisClipboard::clipboardDataChanged so it doesn't need to retrieve the qimage from the clipboard. This means that hasClip() can be true because there are images on the qclipboard, so that needs to be tested.