[WebUI] Fix prefs plugins page not listing enabled plugins correctly

This fixes the display of which plugins are currently running. The old
code was returned a list of enabled plugins containing WebUI code so
switched to calling the entire list of a plugins from core.

Also updated the docstring in json api to reflect actual usage.

deluge/ui/web/js/deluge-all/preferences/PluginsPage.js

@@ -146,8 +146,21 @@ Deluge.preferences.Plugins = Ext.extend(Ext.Panel, {
     },
 
     updatePlugins: function() {
-        deluge.client.web.get_plugins({
+        var onGotAvailablePlugins = function(plugins) {
-            success: this.onGotPlugins,
+            this.availablePlugins = plugins;
+            deluge.client.core.get_enabled_plugins({
+                success: onGotEnabledPlugins,
+                scope: this
+            });
+        }
+
+        var onGotEnabledPlugins = function(plugins) {
+            this.enabledPlugins = plugins;
+            this.onGotPlugins()
+        }
+
+        deluge.client.core.get_available_plugins({
+            success: onGotAvailablePlugins,
             scope: this
         });
     },
@@ -182,9 +195,7 @@ Deluge.preferences.Plugins = Ext.extend(Ext.Panel, {
         window.open('http://dev.deluge-torrent.org/wiki/Plugins');
     },
 
-    onGotPlugins: function(plugins) {
+    onGotPlugins: function() {
-        this.enabledPlugins = plugins.enabled_plugins;
-        this.availablePlugins = plugins.available_plugins;
         this.setInfo();
         this.updatePluginsGrid();
     },

deluge/ui/web/json_api.py

@@ -899,6 +899,16 @@ class WebApi(JSONComponent):
 
     @export
     def get_plugins(self):
+        """All available and enabled plugins within WebUI.
+
+        Note:
+            This does not represent all plugins from deluge.client.core.
+
+        Returns:
+            dict: A dict containing 'available_plugins' and 'enabled_plugins' lists.
+
+        """
+
         return {
             "enabled_plugins": component.get("Web.PluginManager").plugins.keys(),
             "available_plugins": component.get("Web.PluginManager").available_plugins