This is an alternative implementation for issue #12825, replacing commit
424ef17bac, after further discussion with Jacob.
Basically we don't need a new key "gimp:help-id" anymore. Instead, for
every action we add through a "gimp:menu-path" key AND when its name is
prefixed with "gegl:", we generate a help ID from the operation name.
The first few operations in such case already have a help page for them,
with corresponding help ID, in gimp-help repository.
In the future, we may expand this help ID generation rule to any
core operation (i.e. any operation prefixed with "gegl:") but for the
time being, we only do for the ones with a menu path, otherwise we'd
have too many missing pages.
Also fix again gimp_gegl_procedure_get_help_id() so that it searches a
help ID within all existing operations in the "filters" group, as I
realized that this was still broken in some cases.
6076c4d4a6