Support for multi-select in the conversation list.

Unfortunately I'm pretty sure this is the only way to do multi-select on both <> Honeycomb. It'd be nice if we could use the new stuff they were experimenting with in #1601, but I don't think it'll be backwards compatible.
this might be a bit easier to read:
```
MessageRecord message = messageRecords.get(0);
menu.findItem(R.id.menu_context_resend).setVisible(message.isFailed());
menu.findItem(R.id.menu_context_save_attachment).setVisible(message.isMms()              &&
                                                            !message.isMmsNotification() &&
                                                            ((MediaMmsMessageRecord)messageRecord).containsMediaSlide());
```