Only allow moving activity bar icons when editing the bar.

[jmchilton]

Add small visual indicator that they can be moved.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. Try to move the icons outside of the settings panel - notice they don't.
  2. Click activity bar settings - notice the icons can now be moved and have a small dashed border.

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[dannon]

Thanks! Want to target 24.2?

[jmchilton]

@dannon I was going to wait for #19212 to be merged. I've already conflicted it twice and this would again.