Klipper improvements for 5.27

This is effectively the first stage of !2502, and contains the commits from there which are minor enough to make it into 5.27.

Changes:

• Construct HistoryImageItem from QImage, as this allows us to hash the image data directly rather than compressing to PNG and then hashing (which is what we used to do, and is slow).
• Don't save clipboard history when loading it from disk. (Should never have been doing this, but it was a major contributing factor to slow startup times.)
• Make sure that clearAndBatchInsert always clears. (Minor bugfix.)
• Improvements to the above based on feedback (see !2502).