frontend: Fix transition duration not loading correctly

[Warchamp7]

Description

Fixes the transitions duration spinbox to be properly updated when loading data.

Motivation and Context

When a scene collection is loaded, it contains information about the default transition and it's duration. This value was being loaded into transitionDuration and used but because it wasn't using the setter method, the spinbox was not getting updated.

How Has This Been Tested?

Loaded OBS, swapped scene collections.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

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