Added `stylesheet` parameter to VIP themes.

This allows us to keep track of where the theme is installed inside
quickstart and more easily fetch information from its stylesheet at a
later date.
Added `get_theme_action_link` helper function and cleaned up query args.

Previously there were different query args depending on whether or not a
them was supposed to be installed and/or activated. Now the theme to be
processed is passed as the `vipthemehelper-action` with option `install`
and `activate` boolean args.

The `get_theme_action_link` function generates a link that when navigated
to will cause the correct action to be taken.
Using WP_List_Table in the VIP Theme Helper widget.

Looks better and is more organized.
Adding status indicators to VIP Theme Helper table.
Fixing incorrect query parameter in get_theme_action_link.
Update the SVN Credential Form for VIP Theme Helper.

Adding controls so that the user can select whether they want the theme to
be installed and/or activated.