Provide a method to update a current override

Problem/Motivation

An override may often need to be regenerated. E.g.:

• User modifies a component that's included in a feature.
• User exports an override.
• Optionally, user exports the override to a new feature, enables the new feature, and reverts the original component.
• User completes further modifications to the original component.

In this case, there are two different scenarios we need to support:

1. Export just the new modifications to a second override. (This would be needed if, e.g., a site installed a feature that included overrides and then wanted to make and capture its own modifications to the original compontent.)
2. Regenerate the override, so that it includes the previous modifications as well as the new ones.

Currently, the first is supported--it's possible to export new changes. But for the second it appears that one would need to delete the override and then recreate it.

Proposed resolution

• Add a method to regenerate a given override.
• Add a UI to select an existing "overridden override" (that is, a previously exported override that shows new changes) to regenerate.