assistant: pack the sidebar in a frame instead of an event box

assistant: pack the sidebar in a frame instead of an event box

This has mostly two advantages:
- the most obvious one is the theme can render a border around the
  sidebar if it wants to.
- we also can avoid hardcoding a container border width for the sidebar,
  and just use a padding from the theme. This also allows different
  themes to define a different padding, etc.

The drawback is we must draw the background ourselves, but it's easy
enough.

https://bugzilla.gnome.org/show_bug.cgi?id=650530