Added a context menu changing the visible columns in the file browser * files-dock-widget.cc (constructor): add a context to the header; (toggle_headercontextitem_filesize, toggle_headercontextitem_filetype, toggle_headercontextitem_datemodified, toggle_headercontextitem_showhidden): new functions toggling the related entry in the settings file and updating the visibility of the columns (headercontextmenu_requested): new function creating the header context menu; * files-dock-widget.h: new functions toggle_headercontextitem_filesize, toggle_headercontextitem_filetype, toggle_headercontextitem_datemodified, toggle_headercontextitem_showhidden, and headercontextmenu_requested

--- a/libgui/src/files-dock-widget.h
+++ b/libgui/src/files-dock-widget.h
@@ -83,6 +83,13 @@
   /** context menu wanted */
   void contextmenu_requested (const QPoint& pos);
 
+  void toggle_headercontextitem_filesize ();
+  void toggle_headercontextitem_filetype ();
+  void toggle_headercontextitem_datemodified ();
+  void toggle_headercontextitem_showhidden ();
+
+  void headercontextmenu_requested (const QPoint& pos);
+
   /* context menu actions */
   void contextmenu_open (bool);
   void contextmenu_open_in_app (bool);