Multiple selection for Comments&Tags right side bar

When more than one image is selected, comment, rating, date and tags are compared. If a value is different across pictures it will be marked in the UI (grey text). The user can change such a value for all pictures, or leave it untouched.

Additionally, there is a button "More" which allows a batch operation for the currently selected pictures, either Metadata->database or database->metadata (the latter only for multiple selection, for single selection this is equivalent to pressing the apply button).

Problems and pending tasks:
- test it
- the Image Properties tab currently still displays only the first picture. It needs to support a multiple-selection mode with at least the file system properties.
- the "More" button may clutter the UI, better implementations welcome
- in the navigate bar all four buttons are disabled. Perhaps hide them, but without making the whole layout jump (I didn't know how to do that)
- it would be nice if the number in "Editing 25 pictures" could be bold, but KSqueezedText does not allow rich text
- if a tag is added from IPTC metadata and newly created in DB, it is unchecked initially in ImageDescEditTab. Need to debug.
- metadata update notification currently always reloads if no changes are made, and does nothing if modified. For multiple selection more refined solutions are too complex. Perhaps it's ok like this.