[QGIS-Developer] WIP: QGIS Server refactoring - the last one GetPrint

[René-Luc Dhont]

Hi Devs,

I just created a PR for QGIS Server concerning GetPrint.
Because of the lack of test on the different GetPrint specific 
parameters in the precedent versions, I don't know if the refactoring 
gives exactly the same features. So I need reviews and tests to add new 
tests.

The good news for QGIS Server is that when this PR will be merged, we'll 
be able to definitely remove all the old project parser codes from the 
server.

Good Hackfest!
Regards,
René-Luc