* for NB#83601:

* for NB#83601:

with this patch, when we're in low-memory state, we disallow:
- addressbook
- attachments (attaching, opening, saving)
- searching (also over dbus ==> it returns an empty list in that case)
- viewing messages, replying/forwarding

we still do allow:
- composing messages (without attachments)
- saving to drafts (< 25Kb)
- sending (< 25Kb)

also, when sending attachements, make sure the sum of their sizes does
not exceed, if they do, raise an error.

#define MODEST_MAX_LOW_MEMORY_MESSAGE_SIZE (25*1024)
#define MODEST_MAX_ATTACHMENT_SIZE (12*1024*1024)

still TODO is to make sure that the attachment size limitation also applies to
weird filesystems, such as obex and upnp

pmo-trunk-r4405