Building on prior commits, with a few small fixes to them.
First demonstrable changes towards #12605.
Font map plugin is ported as a test case.
Old-style plugins using script-fu-register still work.
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.
