Implementing JGrid in some backend components

This pull request implements JGrid in some backend components.

This touches the users view of com_users and the categories view of com_categories. These changes are 100% backwards compatible and will not influence existing templates. For future templates, it will be easier to modify the existing views and less code will be necessary for this.

This reduces the amount of code necessary for generating the backend tables and simplifies the hathor overrides.

If the platform project accepts a change to JView, the tables could also be modified by a plugin, maybe removing the need for overrides in quite a few cases.