[ACA-2064] support custom icons for extensions (#864)

* icon component, custom svg

* split components, fix modules

* simplify code

* universal icon component

* support custom icon registration

* update docs

* test fixes

docs/extending/extensibility-features.md

@@ -24,6 +24,7 @@ You can create plugins that change, toggle, or extend the following areas:
     - buttons
     - "More actions" buttons
 - Content metadata presets (used on `Properties` tab)
+- Custom icons
 
 Extensions can also:
 

docs/extending/icons.md

@@ -0,0 +1,42 @@
+# Custom Icons
+
+You can register and use custom `.svg` icons with toolbars, context menus, etc.
+
+The icons are declared in the `features.icons` section, for example:
+
+```json
+{
+  "features": {
+    "icons": [
+      {
+        "id": "adf:join_library",
+        "value": "./assets/images/join-library.svg"
+      },
+      {
+        "id": "adf:move_file",
+        "value": "./assets/images/adf-move-file-24px.svg"
+      }
+    ]
+  }
+}
+```
+
+The `id` value must conform to the format `[namespace]:[name]`,
+similar to that of the [Material Icon](https://material.angular.io/components/icon/api) component.
+The icon file path should be relative to the deployed application root (or `index.html` file);
+
+After that, you can use the icon id with other elements, for example:
+
+```json
+{
+  "id": "app.toolbar.move",
+  "order": 500,
+  "title": "APP.ACTIONS.MOVE",
+  "icon": "adf:move_file",
+  "actions": {
+    "click": "MOVE_NODES"
+  }
+}
+```
+
+It is also possible to override the icon value or disable the entry from within external extensions.