Fix bug to check parent file when meta is checked

[StanleyGoldman]

Fixes #913

As a result of an item having it's check state changed in our tree lists, the check state of it's parents or children might need to be changed.

When performing this action from TreeBase<TNode, TData> we need to be sure to call AddCheckedNode(...) or RemoveCheckedNode(...) when those items are not folders. Those functions are overloaded and allow derived classes to set some additional state.

In order to do this cleanly, all nodes check states are now being changed through a central overloaded method.

Unity/src/GitHub.Api/UI/TreeBase.cs

Lines 303 to 328 in d8d92c9

	private void SetCheckStateOnNode(TNode node, bool setChecked)
	{
	SetCheckStateOnNode(node, setChecked ? CheckState.Checked : CheckState.Empty);
	}
	private void SetCheckStateOnNode(TNode node, CheckState setCheckState)
	{
	var isChecked = setCheckState == CheckState.Checked
	|| setCheckState == CheckState.Mixed;
	var wasChecked = node.CheckState == CheckState.Checked;
	node.CheckState = setCheckState;
	if (!node.IsFolder)
	{
	if (isChecked && !wasChecked)
	{
	AddCheckedNode(node);
	}
	else if (!isChecked && wasChecked)
	{
	RemoveCheckedNode(node);
	}
	}
	}

[queerviolet]

Looks good!

I think the multilevel test is would be nice, but if you think it'll be too much of a pain / not worth it, it is definitely not a hill I will die on or ask you to die upon.

[StanleyGoldman]

@queerviolet 🏆. That was a good call. You made me find two bugs.

[queerviolet]

Yay! I have one more test request, then this looks good to go. 🙏