CamelStoreSummary: Move header info to private section.
CamelStoreSummary: Remove CAMEL_STORE_SUMMARY_DIRTY flag.

Just use a gboolean.

Also, the flags value was not used for anything else so remove it.
We still need to load/save a 32-bit value in that spot in the file
to not break binary compatibility, but it's an unused placeholder.
CamelStoreSummary: Move 'summary_path' member to private section.