UI, libobs: Add ability to set custom source icons

[cg2121]

Description

This allows setting dark and light icons for a source.

Motivation and Context

This was a TO DO with the original source icons feature https://github.com/obsproject/obs-studio/commit/f81cd29e0bf49692b172991d85535788dd83ac31

How Has This Been Tested?

Set a custom dark and light icon for a source and switched themes to make sure everything worked.

Types of changes

• New feature (non-breaking change which adds 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.

[cg2121]

Updated to remove the bptr changes.