Set intended filepath in tixiDocument

[merakulix]

Currently it is only possible to set a filepath if the file is written to disk.
This seems intuitive, but in case of working with the tixiDocument in undo/redo situations this means losing access to the filepath while working with the document. See tigl issue 1219.

In between switching the old and new versions of the document the tixi_handle holds the edited information only as a string, until we save it.
In the meantime we still require the filepath, which is, if saved, stored completely in the struct member 'xmlFilename' but not when the document is created by string. There is no other method to set it.

I propose either adding a member to the tixiDocument struct for the intended filepath, or reuse the member 'xmlFilename' and make it clear in documentation, that this can either hold an intended path, only a filename, or a full path(filename inclusive) to a file written to disk.
The new setter function will then either set this intended filepath, which requires also a new getter function, or the 'xmlFilename'. In that case, we could reuse the already existing getter function.