panel, layout: clean up HotCorner handling

Move the HotCorner class from panel to layout, and make the panel
manage its own HotCorner.

Stick the panel's HotCorner into the Activities button actor (rather
than separately floating above it), so that hover tracking on the
button works properly without needing hacks in HotCorner.

https://bugzilla.gnome.org/show_bug.cgi?id=645759