UI plugin dependencies (#4307)

* Add requires field to UI plugin config
* Use defer instead of async for useScript
* Load plugins based on dependency
* Document new field

ui/v2.5/src/docs/en/Manual/Plugins.md

@@ -43,6 +43,10 @@ ui:
   javascript:
     - <path to javascript file>
 
+  # optional list of plugin IDs to load prior to this plugin
+  requires:
+    - <plugin ID>
+
   # optional list of assets 
   assets:
     urlPrefix: fsLocation
@@ -77,6 +81,9 @@ The `exec`, `interface`, `errLog` and `tasks` fields are used only for plugins w
 The `css` and `javascript` field values may be relative paths to the plugin configuration file, or
 may be full external URLs.
 
+The `requires` field is a list of plugin IDs which must have their javascript/css files loaded
+before this plugins javascript/css files.
+
 The `assets` field is a map of URL prefixes to filesystem paths relative to the plugin configuration file.
 Assets are mounted to the `/plugin/{pluginID}/assets` path.