FIX: The "Add Control Scheme..." popup now requires explicit Save/Close user action (case ISXB-1131)

[AlexTyrer]

Description

Previously clicking outside the popup window (anywhere in the editor) would close the popup window.

This would leave a new blank (unnamed, orphaned) control scheme populating the UI - this was confusing.

Now this popup will persist until the user explicitly chooses to Save or Cancel.

Testing status & QA

Manually testing the UI using the reproduction steps provided.
Local and automated testing.

Overall Product Risks

• Complexity: minimal - it's single line change (deletion)
• Halo Effect: minimal - just affects the behaviour of this popup window

Comments to reviewers

This issue manifested when the Input System started using the newer UIToolkit implementation of its UI.

It was a result of fixing another issue in this PR.

Discussions with @Pauliusd01 and @lyndon-unity led to this choice of behaviour change of this popup window:

• it's the least surprising for the user
• it avoids losing any unsaved changes without the user explicitly making a choice.

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • JIRA ticket linked, example (case %%). If it is a private issue, just add the case ID without a link.
  • Jira port for the next release set as "Resolved".
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

After merge:

• Create forward/backward port if needed. If you are blocked from creating a forward port now please add a task to ISX-1444.

[lyndon-unity]

Testing this on Alex's machine shows the issue is resolved, whist the other related issue is not regressed. It also seems a more intuitive UX flow.

[jfreire-unity]

Adding QA

[Pauliusd01]

LGTM. Checked:

• trying to interact with things within the asset while the control scheme popup is open (you can't, It's fully static. Would be great if it blinked when you click outside it just like on Windows, but I don't think that's a thing we can use in UITK)
• entering play mode while it is open (it auto closes and refreshes the window, which I think is fine)
• closing the input window while the popup is still there (same as the play mode scenario)
• basic functionality of adding/removing/editing schemes