Adds option to use the "ShowHiddenFolders" option in .desktop files for sidebar buttons., Ideally this should beas well as a context menu option as well, but at least this gets us a start oentry to toggle it when the option is present in the funcitonality..desktop file.
Included is a small fix for the width of the column display, which now allows for the scrollbar to be displayed properly when the tree does not fit the horizontal area. This is still not perfect though.
It also fixes the "const" keyword having been put in the wrong place.