Plugins aren't refreshed after installing via addon browser

When installing new plugins through the addon browser, the plugin manager (and hence the plugin panel in the prefs dialog) should be told about the plugins.

At present this just logs errors:

Error loading plugin /home/chris/.config/DMDirc/plugins/Server List Support.jar: File does not exist
Error loading plugin /home/chris/.config/DMDirc/plugins/Time Plugin.jar: File does not exist

The files exist on disk with those names.