For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!
And require that showing help is opt-in. Apart from a handful
of built-in providers, most providers will not have help pages
available within the QGIS documentation (including model and
script algorithms). Accordingly, we should hide the help button
by default and only show it for these selected providers.
Note that 3rd party algorithms can still specify custom helpUrl
urls, in which case the button WILL be shown.
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.
