Plugins: Add plugin settings

- Add support for plugin settings, they appear in the same menu as the plugins, indented from the left to emphasize which plugin the setting belongs to
  - Note that plugin settings can still be changed even when the plugins are stopped
- Add tooltips to plugin menu to view the plugin descriptions, description_lang can also be used
- Fix scolling not working on song select when returning from game settings
- Let instance owners set default plugin files in config.py, to make them easier to maintain
- plugins.add() can now add plugins using a url
- Plugins can be hidden from the plugin menu using PluginLoader.hide, an option in plugins.add(), or in config.py
- Make p2.disable() incremental so that multiple plugins can disable multiplayer independently
- Server no longer crashes if certain optional config fields were not copied over from an updated example config
- Fix not being able to unload plugins if one was imported with errors

config.example.py

@@ -13,6 +13,13 @@ ACCOUNTS = True
 # Custom JavaScript file to load with the simulator.
 CUSTOM_JS = ''
 
+# Default plugins to load with the simulator.
+PLUGINS = [{
+    'url': '',
+    'start': False,
+    'hide': False
+}]
+
 # Filetype to use for song previews. (mp3/ogg)
 PREVIEW_TYPE = 'mp3'