Show files in tooltips for pieces progress bars

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.

src/gui/properties/propertieswidget.cpp

@@ -298,6 +298,8 @@ void PropertiesWidget::loadTorrentInfos(BitTorrent::TorrentHandle *const torrent
 {
     clear();
     m_torrent = torrent;
+    downloaded_pieces->setTorrent(m_torrent);
+    pieces_availability->setTorrent(m_torrent);
     if (!m_torrent) return;
 
     // Save path