UI: Adjust styling for Sources list

[Warchamp7]

Description

Minor adjustment to the SourceTreeItem sizeHint calculation and styling rule cleanup.

This also fixes the items in the Sources list being different sizes than the Scene list, which is a bug.

Motivation and Context

There are a few confusing SourceTree specific style rules to account for the fact it uses a QListView with custom widgets instead of a QListWidget. This PR tidies that up a little bit and also removes some unnecessary rules targeting the SceneTree widget.

How Has This Been Tested?

Compiled and checked visual changes.

Types of changes

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

• 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.