example: Move the separator between the header bars

When a separator is between two header bars it will take a special style, better fitting its purpose. This moves the start header bar and the separator following it into a box so they remain a single entity to use in size groups, while making the separator a sibling of the header bar to benefit from this special style.

This supersedes https://source.puri.sm/Librem5/libhandy/merge_requests/88.