optimize icons with svgo using a not-to-destructive options set

Also use it for icons/mageia.png as fallback, though users of common-data
should fix their patches to use mageia-menu.

add new mgabutton svg icon

update all category icons.
For all sub-categories, use the same icon as the main category it belongs to.
Delete a few icons that seem unused, probably more could be cleaned later.

svg files for new icons on main sections.

TODO:
-still 5 icons missing
-export png files
-use main section icons for all sub-sections