Adds option to use the "ShowHiddenFolders" option in .desktop files for sidebar buttons, as well as a context menu entry to toggle it when the option is present in the .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.