new mediamanager

This patch adds a completely rewritten media popup. The following noteworthy
changes were made:

- media manager uses a collapsible namespace tree
- media manager uses AJAX if available
- media manager popup can be kept open when selecting a media file
- only one template is used for the media manager :!: Template
- Editable image metadata is configured in conf/mediameta.php now
- The JS cookie mechanism was enhanced to store key/value pairs
- Language strings can be exported to JS in js.php

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