Switch context menu generation to use a flags enum of actions

[DHowett]

Calling a hundred bespoke WinRT methods to determine what context menu items to show is not scalable!

In the future, we can add...

MenuAction::Copy (applicable when there is a selection)
MenuAction::Paste (applicable when the control is not read-only)
MenuAction::CopyLink (applicable when there is a link!)

I also moved ownership of knowing where the menu was opened to TermControl.
It could go down to Interactivity with some work.
It just felt wrong to have the Core remembering this user interaction state.

I also fixed some trivial resource duplication.

[zadjii-msft]

Is there a reason this is a draft? This seems fine to me?

(I must have missed it when I went on paternity last year, then never saw it again)

[DHowett]

Is there a reason this is a draft? This seems fine to me?

I swear I had a reason. Let me dig back in.

[DHowett]

I think the issue is that I hadn't moved Copy/Paste over to this model honestly