Allow picking the same widget in custom widget gallery

[georgegevoian]

Context

When changing a widget to a custom widget, the gallery is automatically shown, prompting you to select a custom widget. This happens even when the widget type is unchanged (e.g. changing just the select by or group by columns). But since
the primary button in the gallery is disabled if the selected widget is the same as the current widget, you can't confirm
that you'd like to use the same widget; all you can do is dismiss the gallery by closing it, which isn't always obvious.

Proposed solution

You should be able to select any widget from the gallery, even if it's the same widget that's currently in use.

Has this been tested?

• 👍 yes, I added tests to the test suite
• 💭 no, because this PR is a draft and still needs work
• 🙅 no, because this is not relevant here
• 🙋 no, because I need help

[berhalak]

Thanks, nitpicking in comments :)

[paulfitz]

(borrowing this PR for a quick preview, don't mind me...)

[github-actions]

Deployed commit 2cb8d0542f68678840b9a5369ae3651e3391c85b as https://grist-gristlabs-grist-core-widget-save-tweak.fly.dev (until 2024-12-27T20:07:48.072Z)

[github-actions]

Deployed commit 58c9f9ae3d781505ed02a361449f088ad2eee583 as https://grist-gristlabs-grist-core-widget-save-tweak.fly.dev (until 2025-01-02T01:24:29.726Z)