Some changes in the managed_folders handling

[Sergio Villar Senin <svillar igalia com>]

Hi folks,

just FYI, I've just committed some changes in the TnyCamelStoreAccount
and its children. The problem was that the list of priv->managed_folders
was accessed by the children directly and they were not doing it
properly because for example weak references were not properly
unreferenced. In order to fix that two new "protected" methods were
added to the parent to allow children to safely add and remove folders
to the list of managed folders.

New implementations must use those functions. They're located BTW in
tny-camel-store-account-priv.h

Br