header-group: Ref itself instead of header bars

When adding a header bar, ref the header group and connect to 'destroy' signal of the header bar. When a header bar is destroyed or hdy_header_group_remove_header_bar() is called, unref the header bar and remove it from the list.

This way, a non-empty header group only destroyed after every header bar it contains has been removed from the group or destroyed.

Fixes #56 (closed)