Support setting type of the physical structure tree root

[matthias-ronge]

The entirety media unit, i.e. the root of the physical structure tree, must be able to be assigned a type, similar to how it is implemented in the logical structure tree using the document type. In all cases known to us, the type must be called “physSequence”, but it should still be configurable via the rule set, for example like this:

<division id="physSequence" use="physicalStructureRoot">
    <label>Physical sequence</label>
</division>

Rationale: It is insufficient to set this value in the export XSLT, as otherwise no <restriction>s can be enforced on the physical structure tree in the metadata editor, allowing the user to create arbitrary metadata and divisions in it.

Goal: Add support for <division use="physicalStructureRoot"> in the ruleset module. When creating new workpieces, the type should be set automatically.