Adds a new interface QgsAbstractValidityCheck which defines
a single "check" which can be performed on a given QgsValidityCheckContext.
A new application-wide QgsValidityCheckRegistry registers
and manages instances of all known checks, and allows running
of all registered checks of a specific type at once.
Initially the framework is focused toward print layout validity
checks, but the interface has been designed to be generic enough
to allow alternative types of validity checks (e.g. project save
validity checks, processing model validity checks, etc.).
The API is designed to be used both by internal validity checks
and also to be extended by custom, organisation-specific
validity checks. E.g., for print layout validity checks we could have:
