Rigify: Clean up "Rigify Buttons" panel UX

The UX for this panel felt like it could use a facelift. It was extremely ugly to look at, nothing about it was done the correct way and it broke every possible modern Blender UI convention it could.

Before/After:
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After generating a rig:
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- Removed the "overwrite/new" enum.
	- If there is a target rig object, we overwrite. If not, we create. I think that's intuitive behaviour without the extra UI element.
	- If a rig object with the desired name already existed, but wasn't selected as the target rig, the "overwrite" option still overwrote that rig. I don't agree with that because this meant messing with data without indicating that that data is going to be messed with. Unaware users could lose data/work. With these changes, the worst thing that can happen is that your rig ends up with a .001 suffix.
- Removed the "rig name" text input field. Before this patch, this would always rename your rig object and your rig script text datablock, which I think is more frustrating than useful. Now you can simply rename them after generation yourself, and the names will be kept in subsequent generations.

- Renamed the panel from "Rigify Buttons" to "Rigify Generation" in pose/object mode and "Rigify Samples" in edit mode.
- Changed the "Advanced Options" into a sub-panel instead.
- Single column layout.
- Added an info message to show the name of the successfully generated rig:
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Feedback welcome.

Differential Revision: https://developer.blender.org/D11356