Set higher priority for headers and hidden citation overlays.

Set higher priority for headers and hidden citation overlays.

Before the patch, message, headers and hidden citation overlays
had zero priority.  All these overlay have `invisible' property.
Emacs documentation says that we should not make assumptions
about which overlay will prevail when they have the same priority
[1].  It happens to work as we need, but we should not rely on
undocumented behavior.

[1] http://www.gnu.org/s/emacs/manual/html_node/elisp/Overlay-Properties.html