Move to Whats Hot to Overlay infrastructure

Add abstract Overlay and OverlayItem classes to display layers on the
map. This will also be used for subscriptions, search results, etc.
Convert WhatsHot code to use the new Overlay classes.
remove unused code
move menu item handling to overlay
make sure snippet is not null before reading
move onclick handling to overlay
Remove markers on clear()

When the markers are just hidden but not removed, repeatedly searching
for hot locations causes multiple markers to pile up.
move search results to overlay. move all overlays to package
merge with origin master