frontend: Demangle ScreenshotObj and OBSBasic

[cg2121]

Description

This adds a signal to ScreenshotObj, so it doesn't have to directly call OBSBasic.

Motivation and Context

Better code.

How Has This Been Tested?

Took screenshots to make sure they 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]

~~According to https://doc.qt.io/qt-6/qobject.html#disconnect-3, Qt automatically disconnects signals when either of the objects associated with it are destroyed.~~

Nevermind, I get what you mean now, as the signal is in the destructor, and is non blocking, as far as we know.

[PatTheMav]

Please rebase.