This goes with our planned change of not making GimpProcedure arguments order
relevant anymore regarding the PDB API. In particular, it means we don't want to
use GimpValueArray for various procedure arguments API, but directly
GimpProcedureConfig objects.
This change will allow to add or reorder arguments in the future, so that we
won't have to create new PDB procedures when adding new arguments, while still
keeping PDB API stability.
Resolves#8382
Also v2 scripts infer and set sensitivity to drawables
Add two test plugins clothify-v3.scm and test-sphere-v3.scm.
Temporary, to be removed when 3.0 ships.
Some refactoring (extracting methods, moving functions to new files).
Some drive-by fixes to script-fu-arg.c revealed by using GimpProcedureDialog.
