Blender Git Loki

Asset Browser: Context menu for catalogs

The context menu is a standard way to expose operations of the clicked
item to the user. They expect it to be there, and we can make use of it
as a place to put more advanced operations in.

The menu contains:
* New Catalog
* Delete Catalog
* Rename

Also removes the 'x' icon to delete a catalog from the right side of a
row. This was just placed there temporarily until the context menu is
there. It's too easy to accidentally delete catalogs with this.