Update gesture code to check for `IContextMenu`

[maribethb]

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Problem

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Request

The current gesture code currently checks a specific set of objects and calls the showContextMenu function for the object if it is what was right-clicked on.

Instead, the gesture code should:

• Check what is currently focused (this assumes that Make "right click" change the focus to the thing that was right-clicked on #8840 is complete)
• Check if the currently focused thing implements IHasContextMenu
• If it does, call its showContextMenu function and stop propagation of the event. If it doesn't, do nothing and don't stop propagation since the click wasn't handled.
• The current calls to bringBlockToFront and hideChaff should be moved into their respective objects' implementations of showContextMenu since the handling depends on that specific object, and there's no reason for the gesture code to know about it.

Alternatives considered

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Additional context

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[maribethb]

I'm no longer sure we should do this, at least right now. The gesture code basically can't be triggered from an arbitrary object because each individual object starts the gesture by calling a method like handleFlyoutStart that sets the context of the gesture. It doesn't make sense to change that just for the handleRightClick method. Possibly in the future Gesture should be redesigned to work with focus instead of having to know about each individual type of thing that might get a gesture on it, but that's a big project.

I think the last bullet point is more true still, but the rest of the gesture code also calls bringBlockToFront when a gesture happens on a block (or a thing within a block). I'm not totally sure that the gesture code should be the one responsible for this, but again since it's everywhere in the class, should maybe be done as a bigger refactoring

In any case, this isn't essential to getting the new context menu features working so it's safe to punt on this. You already can't right-click or any other gesture on arbitrary things (not without adding code to the gesture class). We can still have the context menu open via keyboard nav on other non-clickable elements like connection because that won't be connected to the gesture code.

[gonfunko]

See also #3568, which could be resolved this way.