Changes only to ScriptFu.
The third term (the default) of a SF-FONT etc. spec is now ignored.
Test case is SF>Test>Sphere. There are still crashing issues
related but separate.
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.
