Updated prefs backend storage documentation

Does IMSP even exist anymore? Regardless, no need to prominently
advertise that support anymore.
Add IMP_Script_Package_DynamicBase
[mms] Add 'dynamic_prefs' hook to allow for changing of default values for browser-based preferences in dynamic view.
[mms] Fix displaying Virtual Trash mailbox (Bug #13418).

Conflicts:
	imp/docs/CHANGES
	imp/package.xml