Spot/469 (simplify plugin manager)

Addresses Issue #469:

* Remove the "menus" panel (this can be set from the prefs file).
* Get rid of the text field next to "Add plugin from URL".
* When a user clicks one of the plugin buttons, it just loads.  It has a status light that cycles from gray to yellow to green.
* The "Add plugin from URL" button becomes just another button in the big list (on the far right) with label "Open from URL...", which opens a field where you enter the URL and confirm.
* change the accordion list into tabbed panels.  plugins with no parameters do not create a panel.