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.
Why: puts most methods for SFArg (a struct) in one place, for ease of maintenance.
Prepares to fix issue 8328. Prepares to make SF use GimpProcedureDialog.
Mostly moving code, with no intended change in functionality,
except fixed an property.nick for an arg is now what a script author provided,
instead of generated.
All internal to libscriptfu. No changes to the exported API or to i18n.
Lightly tested, since more substantive changes coming for issue 8328.
ScriptFu>Test>Sphere is the test case.
