Also refactoring: extract two functions
Add test case scripts/test/register-fail
Catches more registration errors.
Any errors not caught at least will not crash.
Remains to be ported to 2.10, but only a few lines, without the refactoring.
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.
