delegates: Fix thumbnail placeholder icon position

The thumbnail placeholder, shown when the item has no previews, assumed the thumbnail would cover the whole delegate, so when it's not the case, it can show in unwanted places.

To fix this, add a thumbnailArea property which allows the derived delegates to set it where the actual thumbnail is placed.

The previous item named thumbnailArea is renamed to contentArea to better reflect what it does and avoid confusion.

It is easier to test this on a category which don't usually have preview thumbnails, such as Kalzium molecules:

	BEFORE	AFTER
Tiles
Icons (no change here)
Large Previews