frontend: Make OBSAbout self contained

[cg2121]

Description

This makes it so OBSAbout isn't tangled with OBSBasic.

Motivation and Context

Make frontend less messy

How Has This Been Tested?

Opened about dialog to make sure it still worked

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

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]

It is needed for the GetDataFilePath function. OBSApp.hpp was included with OBSBasic.hpp before.