data defined buttons for model child algorithms
The context was not previously exposed, so users would not have
been aware that they can utilise all the variables and functions
available to parameters within child algorithms.
A new widget which handles parameter values for child algorithms
within a model. Instead of the previous approach of requiring
individual widget wrappers to handle creation of a suitable
model widget, we do all this automatically for them.
This widget uses a stacked widget with a toolbutton to select
the parameter's source, instead of the previous combo box approach
(which didn't scale well for large models). I.e. users select
first whether the value is taken from a static value, a
model input, or an output from a different child algorithm. The
widget then changes appearance and behavior based on this
choice.
Additionally, a new option is present for all parameters
of using a "precalculated expression". This expression is
evaluated once before the child algorithm is executed
and used during the execution of that algorithm.
ModelerParametersDialog and ParametersPanel have to keep list of
wrappers only. widget and label( if needed) are created through
WidgetWrapper.createLabel()
their unique id()
This is used when generating the QgsHelp url for algorithms
attached to the providers.
Implement helpId overrides for the native and 3d providers so
that they return 'qgis' helpIds, meaning that all QGIS processing
algorithm documentation can be kept within the same url path
regardless of which QGIS provider library it sits within.
This also allows us to freely move algorithms from the Python
'qgis' provider to c++ 'native' provider in future releases
without breaking the help URLs.
Fixes#17231
Check that all parameter's values pass the validity check,
even if not returned as QgsProcessingModelChildParameterSource.
In case of list, tests that it is really a QgsProcessingModelChildParameterSource list,
and create a QgsProcessingModelChildParameterSource from the list if it is not the case
(useful for custom parameters that return lists as ParameterFieldsMapping).
The previous naming was too easily confused with processing outputs.
Rename them to QgsProcessingParameterFileDestination, etc... to
make it clearer what they are used for.
So that multiple input type parameters are correctly handled,
allowing models with child algorithms like merge to correctly
use any combination of static layers/model inputs/child outputs
as their input parameter
