UI: Make list widgets look consistent

[cg2121]

Description

The scene and source list widget items were different heights. This sets a height in the themes instead of padding. The source tree delegate is also removed in this code.

Before:

After:

Motivation and Context

Make UI look better

How Has This Been Tested?

Looked at each list widget to make sure they looked good

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Code cleanup (non-breaking change which makes code smaller or more readable)

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.

[gxalpha]

Did you ensure that past issues with gaps between sources in that list didn’t come back? In particular, if you dragged another source into the gap, it would end up at the bottom (See also https://bugreports.qt.io/browse/QTBUG-106395).