include non-editable attributes in export

[mfinger-irt]

Some attributes like origin/position have different metrics available but currently only % is supported. Is a document loaded with e.g. origin=center, it will not be editable in the editor and it will also not be exported correctly when the file is saved.

[andreastai]

The metric when a file is loaded should not be a problem. It will be normalized when the IMSC document is parsed for the first time. @tts:origin can not be set to "center". This is value for tts:position.

It is true that for editing only limited sets of length metrics are supported. For the moment it makes sense to keep it that way.

Not all TTML style attributes can be edited using the imscedUI but in recent commits, the export of these attributes has been added. I will check again if still some attributes are missing.