Search improvment suggestion

[Emmanuel <e allaud wanadoo fr>]

	Hi all,
To fix the performance problem of the search function we could just try to 
make libbalsa_mailbox_get_matching smarter : it should store the matching 
messages and recalculate it only if the mailbox content has changed.
I can make a patch if this fix is OK for at least someone else ;-)
Bye
Manu