UI: Cleanup frontend event handling

[cg2121]

Description

This adds a function to OBSBasic to call on_event, so every time a event is called, the api variable doesn't have to be checked every time.

Motivation and Context

Code cleanup

How Has This Been Tested?

Tested events with one of my plugins

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.