broken round tripping via `filter_get.xsl` and `filter_put.xsl`

Just recognised while reviewing #89 and we've been there before with #66 :

These scripts are hard to understand and most code seems not to be called at all
More severe: filter_get.xsl transforms the MEI input file in various ways (adding "default" attributes with empty values, removing the music section, transform some elements to HTML), which are only in parts undone or reconstructed by its counterpart filter_put.xsl. E.g. the example file "nielsen_maskarade.xml" features some <mei:rend rend="sup"> in <mei:identifier>s which are transformed into <html:sup>. This is not re-transformed by filter_put.xsl, so modifying (and saving) that file with MerMEId (at any location) will write that <sup> into the MEI file (as string!).

I think the best way forward would be to rewrite those files and

only transform MEI elements into HTML when those are input of the TinyMCE editor
add a proper wrapper/attribute breadcrumb for those changes so we can filter them out later
the XML display of the files (accessible from the main list) should display the unfiltered file. For use with orbeon we should add a URL parameter (e.g. mode=orbeon) to filter_get.xsl. NB. It might be easier to add a URL parameter for the "raw" view, but I think of the "raw" view as the default ant the "orbeon" view as the special case.

Edirom / MerMEId