add shared hover state in session model

[rbuels]

Currently we have session.selection, which is the currently "selected" object, which can be anything.

Let's add a session.hover, which is the currently hovered-over object, which can also be anything.

One example use case would be when hovering over a spot in the LGV, the LGV would set hover to a location object that specifies that the user is hovering over a specific spot in a specific ref seq in a specific assembly.

Also add a couple of integrations for this to drive development:

• LGV both sets and pays attention to session.hover. Draws cursors in the track panel and the overview in response to session.hover, and sets session.hover when the user mouses over each of them.
• Ideogram plugin does the same: sets and reacts to session.hover

[cmdcolin]

was added in #4013