I'm looking to initially "Open - Edit - Save" XML files, and noticed there was no TextMaker "Save As" to .XML format. The example of the open-source contacts-app is only one example of how more & more applications use XML as its preferred file extension. Again, as an example, major database program acronyms now seem to be either xml, dbf, and/or sql with csv as a minimum import/export filter, not to mention ODBC for universal SQL-connectivity.
I am really struck by how TextMaker now has the editing & formatting tools, but; only needs "the interface" and "save-as" filters, literally "re-packaging" the components that already exist in TextMaker. I can think of 3-4 interfaces & filters that would replace other outside applications -- Perhaps as TextMaker Plug-Ins -- With XML editing and ePub editing as just two of them. Familiarity with TextMaker really makes it a desired solution for editing.
Personally, a more immediate use would be to edit and/or construct ePub documents -- Which can even be made without an actual ePub application if you have the right template. ePubs only technically need the correct directory structure for the ePub Reader, expected text-type file extensions (i.e. html, xml, css, opf, etc.), and optional graphics (including a suggested cover of 412x616 pixels). ePubs also need "a means to package" its directories to an uncompressed zip-file then to renaming that zip-file extension to the uncompressed epub-file extension -- For reading by apps like Calibre, Firefox ePub Reader, etc.