If tooltip text contains an empty HTML body, Qt still shows it as an
empty rectangle. Thus, output HTML tags only if we are within the
image region, and return true empty string if we are at the borders.
In addition to the current tooltip, which shows color legend, if user
holds the Shift key during hovering we show another tooltip which
contains a table of contents for the piece under the moue cursor. The
table lists file sizes and names. If the cursor points to a part of a
file which spans several pieces, those pieces are highlighted.
