Move all Graphics applications into their own top-level category

[baimafeima]

Graphics is a separate top-level category in the Budgie menu, following the freedesktop specifications. I think it would be helpful to create a separate Graphics top-level category in the Software Center to make this more consistent instead of having a top-level category Multimedia & Graphics with Graphics as a sub-category.

[ikeydoherty]

Graphics is a form of multimedia anyway. Note our components aren't tied in any way to the XDG Menu Specification because said menu spec is completely on crack.