Use relative positioning for category popups, move out of overlay

Instead of putting these popups in an overlay, put them
in the category widget. This makes scrolling work more
nicely, and makes things easier to deal with in general.

This requires that the popup position itself using
getRelativePosition(), because it's no longer in an
overlay. This also means these popups should now position
themselves correctly no matter where they are.

Change-Id: I09a1e5891a897d634c41d386a2307fe3df2a9157
Merge "Use relative positioning for category popups, move out of overlay"