This allows optional outputs (such as null geometry features detected
by the Remove Null Geometries algorithm) to be skipped by default
when desirable.
The previous naming was too easily confused with processing outputs.
Rename them to QgsProcessingParameterFileDestination, etc... to
make it clearer what they are used for.
Also fix some UX issues with the panel in folder, such as
correctly remembering/restoring last used folders, defaulting
to home dir, don't clear value when canceling while browsing
This adds a lot of flexibility to algorithms, as it makes output
sinks truely optional. For instance, the various "Extract by..."
algorithms could add a new optional sink for features which
'fail' the extraction criteria. This effectively allows these
algorithms to become feature 'routers', directing features onto
other parts of a model depending on whether they pass or fail
the test.
But in this situation we don't always care about these failing
features, and we don't want to force them to always be fetched
from the provider. By making the outputs truely optional,
the algorithm can tweak its logic to either fetch all features
and send them to the correct output, or only fetch
matching features from the provider in the first place (a big
speed boost).
