Fix data-focus updating when exit is blocked

[Murreey]

Description

Refactoring how data-focus is applied again. Previous fix in #38 still wasn't quite right, as it changed the attributes before the blocked exit checks happen.

• Moved the storing of the element we're leaving higher up
  • Mostly for clarity and to decouple it from bestCandidate logic
  • Also now checks that elem is actually a focusable, so it doesn't store IDs of containers or potentially any element that's been passed in
• Moved the updating of the candidate container to after the between-container movement logic has happened
  • Doesn't have to worry about overwriting the old value before it's needed
  • Happens after data-block-exit has been checked

Motivation and Context

Fixes a bug where if movement into a new container is blocked by data-block-exit, it still gets its data-focus modified to the candidate that it was going to try to move to. The focus should only update when it actually moves.

How Has This Been Tested?

Existing data-focus tests still pass, added a new test for this specific behaviour. Checked that it fails on master, passes on this branch.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.