UI: Use OBSSourceLabel for item widget labels

[cg2121]

Description

This changes the labels for the source tree/visibility item widgets to use OBSSourceLabel, as it handles the renaming of sources.

Motivation and Context

Less code

How Has This Been Tested?

Renamed sources in the filters dialog and the source tree

Types of changes

• 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.

[cg2121]

Updated to latest master