Video tag in TinyMCE not visible when editing

Description

When inserting a video tag manually in an XHTML property, the video isn’t visible in TinyMCE.

When previewing, the video is rendered. But when editing, the img placeholder is rendered with a transparent background and is impossible to see since the css classes uses for indication are not defined.
The css classes (mveItemMedia, mceItemVideo, etc.) must be defined in the content.css used when editing.
The media dialog is also missing localizations for selecting media type.

Steps to reproduce:
1. Insert a video in tinyMCE with the media dialog.