plugin manager improvements

[reacocard]

Collection of a few needed improvements to the plugins manager:

 * There needs to be a way to distinguish between system-wide plugins and per-user plugins
 * For per-user plugins, there needs to be a way to uninstall them from the gui. Uninstalling from the gui should also disable the plugin, so that if the plugin is also available system-wide it does not get reenabled unexpectedly.
 * When installing plugins, we should consider warning the user about the potential risks of running untrusted code. If such a dialog is implemented, it should have a checkbox to disable it permanently (unchecked by default).
 * Because there are now so many plugins in Exaile, sorting them into categories might be useful and make it easier for users to select the plugin most appropriate for their situation.

[Steve Dodier-Lazaro (sidi)]

One more thing that needs to be done is to make sure that when the user
resizes the preferences window, the extra space goes to the plugin list and
not to the description area.

[Mathias Brodala (mathbr)]

The size gained through dialog resize is given to the list with r3219.

[Dustin Spicuzza (dustin-virtualroadside)]

I really like the idea of sorting the plugins into categories. In fact, I had just thought of that independently. Expect that to make it into trunk this week.