Implement sane behavior of space key in torrent content list. Closes #140.

Definitions:

Selection in QTreeView consist of two things:

   currentIndex -- is a (dotted) cell where user clicked last time. Note
                   that it is a cell
   selectedIndexes -- is a set of cells (blue) of current selection.

Checkboxes in torrent content lists are belong to COL_NAME column.

Problem:

The problem is that spacebar toggled checkbox only in currentIndex
index. This has two consequences:

  1. It is impossible to toggle checkboxes on multiple rows
     simultaneously.
  2. If currentIndex is not in COL_NAME column space doesn't work. This
     problem is amplifyed by the fact that SelectionBehavior is set to
     SelectRows. So visually it is impossible to tell which column does
     it belong to. For end user it looks like "space doesn't work
     sometimes".

This patch addresses the problem by implementing TorrentContentTreeView
derived from QTreeView and overridding keyPressEvent(QKeyEvent*).

The code of TorrentContentTreeView::keyPressEvent is written under
inspiration from QAbstractItemView::keyPressEvent and
QItemDelegate::editorEvent.