UI: Fix source tree icon spacing

[Warchamp7]

Description

Fixes the spacing on source tree icons to be consistent and properly sized.

Motivation and Context

More consistent and proper spacing of all icons in source list entries. This had more averse effects on the Classic theme due to it inheriting from Yami now.

Before

After

Before

After

How Has This Been Tested?

Tested on Windows

Types of changes

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

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

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.