frontend: Move source selection handling to parent

[Warchamp7]

Description

Moves the calls to update various UI elements in OBSBasic from the SourceTreeItems up to SourceTree itself.

Fixes #13026

Motivation and Context

Currently certain parts of the UI are updated whenever an individual SourceTreeItem is selected or deselected. This leads to the bug in #13026 as the item gets removed from the source list before it can call Deselect.

This PR moves the UI update calls to the event in the SourceTree itself instead. This also has the side effect of removing some redundant calls to UpdateContextBarDeferred and UpdateEditMenu, though the former is deferred anyway.

The fact that SourceTreeItems even hold a reference and make direct calls to the SourceTree is bad. The entire event hierarchy here is inverted with a lot of redundancy and arcane managing of libobs. It should be cleaned up in the future.

How Has This Been Tested?

Selected multiple items, deselected multiple items, both in and outside of groups, including mixed selections.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

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.

[PatTheMav]

It's logically more correct. Haven't run in-depth tests whether this change to the better has other unexpected side-effects though.