MX21, XFCE fixing and improvement

MX 21 improvement and fixing

-> fix the dimension of opening dialog windows whose bottom is presently going 2 rows under the screen bottom;

-> put a second confirmation for “alphabetical order” upon clicked the “desktop->sort icon”;

-> remove the menu point “remove item” … scrambling the whole panel;

-> add a history/wayback for desktop layout;

-> uniform the menues wisker and rx-mouse, editable with a “a la carte” menu editor;

-> include a sort of “fence” by creating “desktop category folders” (not Thunar) within those include “iconed launchers”.