Fix segfault when navigating to a message whose metadata wasn't loaded yet

fixes #594
Don't loop through the input when parsing e-mail addresses in the recipients widgets

refs #595
refactoring: don't call something a "parent" when it isn't a parent at all
BUG: don't break the recipient parsing on empty e-mails
Composer: try to pick the sender identity based on the recipients/senders/... of the original message
xtuple: prevent segfault when accessing argv out-of-bounds
xtuple: change the option for specifying the password on the command line to -w

This is required due to compatibility concerns with how the man XTuple GUI has
always worked. The fact that Trojitá the GUI has always used "-W pass" is
apparently irrelevant :).