Torrent properties dialog improvements

Simplify DND checkboxes drawing, this also fixes incorrect drawing on
Mac when file tree widget is inactive.
Do better job calculating column widths for file tree to avoid ellipsis.
Fix file tree sorting order for size and priority columns.
Change key to toggle priorities to Shift+Space instead of Enter/Return
to avoid conflicts with name editing and default button handling.
Fix selected tracker item background drawing in certain cases.

qt/FileTreeView.h

@@ -44,8 +44,9 @@ class FileTreeView: public QTreeView
     void openRequested (const QString& path);
 
   protected:
-    // QObject
-    bool eventFilter (QObject *, QEvent *);
+    // QWidget
+    virtual void resizeEvent (QResizeEvent * event);
+    virtual void keyPressEvent (QKeyEvent * event);
 
   private:
     FileTreeModel * myModel;