Use the control that requested the context menu, instead of the active one

[zadjii-msft]

As mentioned in #15760

When you right-click on a non-active pane, it becomes active, but the context menu may be displayed before this happens, thus showing the Restart Connection item based the wrong pane's status.

As far as I can see, when a pane is (right)clicked:

1. If unfocused, Focus is called. This goes through the GotFocus handler which eventually calls tab->_UpdateActivePane(sender);
2. PointerPressed is raised which eventually shows the context menu

The first point is done asynchronously, so may update the active pane too late when the menu is already displayed (despite both end up in the UI thread).

To fix this: we plumb the control that the context menu was opened for all the way through to where the event is actually handled (in _PopulateContextMenu)

• Tested manually