obs-frontend-api: Add theme functions

[cg2121]

Description

This adds a frontend function to check if a theme is dark and also adds an event for when the theme is changed.

Motivation and Context

If a plugin uses their own icons, for example, they can determine whether to use light or dark ones.

How Has This Been Tested?

Tested with one of my plugins.

Types of changes

• New feature (non-breaking change which adds 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.