frontend: Enable use of delete key on macOS

[PatTheMav]

Description

Adds the "Delete" key to list of keyboard shortcuts that can trigger a scene item removal on macOS.

Motivation and Context

While it is canonical to use the backspace key as an alternative to the dedicated "delete" key (which is omitted on many smaller-sized Apple keyboards), the delete key is still available on full-size Apple keyboards and obviously third-party keyboards.

This change adds the delete key as an alternative to the backspace key to allow removal of scene items from the scene list in the UI.

How Has This Been Tested?

Tested on macOS 26 with full-size Apple keyboard.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.