Editor stylesheets does not include stylesheets for subplugins

Moodle has support for two types of stylesheets in some plugins:

• styles.css
• editor_styles.css

For editors which use a iframe for the editor content, we cannot just use the standard styles.css (boost and all) because it will bring in too much for the editor (things like background, margins, and so on). Instead we support an editor_styles.css sheet which has a minimal style set supplied by the theme, and editors.

I discovered whilst implementing MDL-75071 that we currently do not generate editor CSS content for editor subplugins. This means that a plugin (such as the h5p plugin) is unable to set the style of the something like the h5p placeholder within the editor, and doing so requires changes to the editor's own editor_styles.css which would be a breach of component communication principles.

This change adds support for subplugins to also provide their own editor_styles.css which will also be included and applied in these situations.