When tag filter is active, indicate how many photos are hidden

If any photos are hidden by the tag filter, a simple non-interactive note at the bottom of the thumbnail list should say how many.

• It should be invisible if no photos are filtered out.
• If all photos are filtered out, the note should still be visible (as the only thing inside the thumbnail list) .
• It should not react to any input or mouseover (careful if e.g. tag panels or thumbnail GUI assume that anything inside the thumbnail list is a thumbnail; for these purposes, the note should behave like the list background).
• It should be visible below the last photo thumbnail, even if there are still enough photos satisfying the filter to fill the whole list from top to bottom -- i.e., if the thumbnail list is "full", but the note is supposed to be displayed, the note should "expand" the list's inner size.