ECalBackendStore: Add a "timezone-cache" construct-only property.

ECalBackendStore: Add a "timezone-cache" construct-only property.

Instead of keeping its own an internal hash table of icaltimezones, have
ECalBackendStore take an ETimezoneCache in e_cal_backend_store_new().

Usually the ETimezoneCache will be an ECalBackend, which implements the
ETimezoneCache interface and owns the ECalBackendStore.  For that reason,
the store only keeps a weak reference on its ETimezoneCache to avoid a
reference cycle.

New functions:

    e_cal_backend_store_ref_timezone_cache()

This is an API break.  3rd party backends will have to be adjusted.