Previously this was only enabled for optional arguments (i.e.
those with default values). Enabling them for all arguments
allows for more readable PyQGIS code, and there seems no
downside given that we already have this support partly enabled.
The consequence of this change is that when 3.0 API is frozen
the freeze must also include the naming of function arguments,
since that's effectively now part of public API.
2b7b0db747