Rigify: Allow entering name on first generation

Requested by @dexon, see rBAece39d809ce#339214.

This patch tries to improve a workflow that was hurt by D11356.
One of the goals of that patch was to make the connection between
metarig and generated rig stronger and more reliable, by using
datablock pointers rather than text-based matching. This allows
users to rename their datablocks as they would do with any other
datablock in Blender, so there's much more freedom with naming
when working with one metarig per generated rig per file.

However, this made it hard to duplicate the metarig, make changes
to it, and make a new rig out of it within the same file. Renaming
several datablocks in this case is not an "option", but an unintuitive
necessity.

With the patch applied, when the metarig has no target rig, there is
an option to input a rig name. If something is input by the user, that
name will be used for the rig, the widget collection (by extension the
widget objects) and the rig script, as long as they are not specified.

So, while still requiring a few more clicks than before, the workflow
for duplicating metarigs to make new ones is now much more reasonable:

- Duplicate metarig, make changes
- Remove the datablock references
- Input a rig name
- Generate

If the a rig object with the same name already existed, it will NOT be
overwritten, and the generated rig will have a .001 suffix. This matches
the "new" option of the removed "new/overwrite" options.

Meanwhile, the other workflow of having one metarig per generated rig
per file is not affected at all: The text box for the name input can
simply be ignored on the first generation. It will not show up after
that, since you would never remove the target rig in this workflow.

Differential Revision: https://developer.blender.org/D14757