Rework: Remove XML storage

[PhMemmel]

Currently GGB files are being stored both XML and base64 encoded. Using XML has the disadvantage of not being able to store background images etc.

Therefore, we should verify if the XML is really needed or - whereever it is being used - if it can be subsituted by the base64 encoded content and remove the storage of XML completely. This would also avoid possible inconsistencies as currently we are storing the same data twice.

Removing XML might require an investigation if there is a possibility of questions only stored as XML. In this case we should consider implementing an upgrade step verifying if there is a base64 encoded value and, if not, converting the existing XML to base64 and store it

[PhMemmel]

Looks like we need to keep the XML in the DOM to extract data from the GGB applet from PHP side after form submission. As far as I can tell, we can drop the database field and rely on storing it as base64.

Another approach would be to handle all the extraction of information from the GGB applet by JavaScript and just store the extracted variables into the DOM which then is being extracted by PHP. This would require a complete rework of the validation and parameter saving logic though.

[PhMemmel]

Discussed internally: We are removing XML completely on PHP side and go for the JS approach: Extract all necessary information via GGB JS API, put them into hidden fields and do the verification of these params directly in PHP.

While doing so we need to figure out if there are parameters we need to separately save to the database, eventually we will have to add another field for that. Ideally we can avoid that and save all necessary information directly via question api. As we have stripped off all unnecessary information by avoiding the XML we might be able to do so, will see while implementing.