Allow adding custom CSS to editor context menu

[jswiderski]

Type of report

Feature request

Provide description of the new feature

Contents of editor menus, including context menu are being displayed inside an iframe. For context menu only editor.css file is being attached.

Two approaches can be used here:

1. Attach contents.css to context menu iframe just like it is being done for dropdowns.
2. Add configuration setting for specifying path to custom CSS file for editor context menu.

From what I have checked, the whole logic works in the following way:

1. Everything starts with panel prototype using CSS:
   https://github.com/ckeditor/ckeditor-dev/blob/major/plugins/panel/plugin.js#L114-L117

2. That CSS gets inserted into array which later gest inserted into iframe:
   https://github.com/ckeditor/ckeditor-dev/blob/major/core/tools.js#L346-L360
   https://github.com/ckeditor/ckeditor-dev/blob/major/plugins/panel/plugin.js#L75-L80
   https://github.com/ckeditor/ckeditor-dev/blob/major/plugins/panel/plugin.js#L114-L117

3. Context menu plugin is based on general Menu plugin which uses panel mentioned in point 1 to specify its own CSS:
   https://github.com/ckeditor/ckeditor-dev/blob/major/plugins/menu/plugin.js#L163
   for which path is calculated based on getCssPath method:
   https://github.com/ckeditor/ckeditor-dev/blob/major/core/skin.js#L137-L163

It seems to me that the best place for inserting new CSS would be here:
https://github.com/ckeditor/ckeditor-dev/blob/major/plugins/menu/plugin.js#L163
We could create a new configuration setting which would get read there.

[jacekbogdanski]

Actually, this feature request seems much easier to implement than digging so deep into contextmenu dependencies. It just requires passing additional parameter into context menu base constructor call. I will create following PR with an improvement.