pad.html: for each client plugin, add a class to #editorcontainerbox

This commit implements the following behaviour:

1. adds a function clientPluginNames() to hooks.js (mimicking what is done in
   static.js), which returns an array containing the list of currently installed
   client side plugins. The array is eventually empty.

2. calls that function in pad.html at rendering time (thus server-side) to
   populate a class attribute.

Example results:
- with no client-side plugins installed:
  <div id="editorcontainerbox" class="">

- with some client-side plugins installed:
  <div id="editorcontainerbox" class="ep_author_neat ep_adminpads">

Looking at the existing code (src/node/hooks/express/static.js#L39-L57), a
client-side plugin is defined as a plugin that implements at least a client side
hook.

NOTE: there is currently no support for notifying plugin removal/installation
      to the connected clients: for now, in order to get an updated class list,
      the clients will have to refresh the page.

Fixes #3488

doc/plugins.md

@@ -47,6 +47,22 @@ You can omit the `FUNCTIONNAME` part, if the exported function has got the same
 ### Client hooks and server hooks
 There are server hooks, which will be executed on the server (e.g. `expressCreateServer`), and there are client hooks, which are executed on the client (e.g. `acePopulateDomLine`). Be sure to not make assumptions about the environment your code is running in, e.g. don't try to access `process`, if you know your code will be run on the client, and likewise, don't try to access `window` on the server...
 
+### Styling
+When you install a client-side plugin (e.g. one that implements at least one client-side hook), the plugin name is added to the `class` attribute of the div `#editorcontainerbox` in the main window.
+This gives you the opportunity of tuning the appearence of the main UI in your plugin.
+
+For example, this is the markup with no plugins installed:
+```html
+<div id="editorcontainerbox" class="">
+```
+
+and this is the contents after installing `someplugin`:
+```html
+<div id="editorcontainerbox" class="ep_someplugin">
+```
+
+This feature was introduced in Etherpad **1.8**.
+
 ### Parts
 As your plugins become more and more complex, you will find yourself in the need to manage dependencies between plugins. E.g. you want the hooks of a certain plugin to be executed before (or after) yours. You can also manage these dependencies in your plugin definition file `ep.json`:
 

src/static/js/pluginfw/hooks.js

@@ -125,3 +125,29 @@ exports.callAllStr = function(hook_name, args, sep, pre, post) {
   }
   return newCallhooks.join(sep || "");
 }
+
+/*
+ * Returns an array containing the names of the installed client-side plugins
+ *
+ * If no client-side plugins are installed, returns an empty array.
+ * Duplicate names are always discarded.
+ *
+ * A client-side plugin is a plugin that implements at least one client_hook
+ *
+ * EXAMPLE:
+ *   No plugins:   []
+ *   Some plugins: [ 'ep_adminpads', 'ep_add_buttons', 'ep_activepads' ]
+ */
+exports.clientPluginNames = function() {
+  if (!(exports.plugins)) {
+    return [];
+  }
+
+  var client_plugin_names = [...new Set(
+    exports.plugins.parts
+      .filter(part => part.hasOwnProperty('client_hooks'))
+      .map(part => part['plugin'])
+  )];
+
+  return client_plugin_names;
+}

src/templates/pad.html

@@ -100,7 +100,7 @@
             <% e.end_block(); %>
         </div>
 
-        <div id="editorcontainerbox">
+        <div id="editorcontainerbox" class="<%=hooks.clientPluginNames().join(' '); %>">
             <div id="editorcontainer" class="editorcontainer"></div>
             <div id="editorloadingbox">
               <div id="passwordRequired">