UI: Remove hardcoded list widget spacing

[cg2121]

Description

Not all list widgets were using this consistently, and these should be set in the themes instead.

Motivation and Context

Makes things consistent

How Has This Been Tested?

Looked at each list widget to make sure there was no extra spacing besides margins set in themes.

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.

[Warchamp7]

Default values can vary by operating system, so I'm a bit hesitant to remove hardcoded values. I'd rather ensure they're consistent everywhere.

In some places it's also difficult to replicate spacing behaviour using only QSS since the only real method is margin, and Qt has a lack of or inconsistent implementation of things like first-child / last-child.

[cg2121]

Updated to set the list widget spacing to 1 for all list widgets.