Check if project has CRS assigned

[wonder-sk]

If a QGIS project has no CRS defined, things will very likely not work correctly in Input (and many thing won't work properly in QGIS either). We should check for that and issue a warning in project validation if that is the case.

In addition to that, it would be good to check that also none of the project layers have invalid/unknown CRS, and have a warning in validations, as those layers may not show up correctly...