Bug 1067228 - [Messages][Refactoring] Move subject management to a separate component. r=julien

I still have issue here, previously in this case subject was removed and focus moved to composer input, current patch broke this behaviour. Trying to figure out how to fix this at the moment, initial thought was to manipulate with focus in ```visibility-change``` event handler only in Compose, but ```subject.hide``` is called in ```Compose.clear``` that's not desired.