Choose the message to reply to based on selection

If part of a message is selected, that will be the replied-to message.
If parts of multiple messages are selected, or nothing is selected, the
last email will be replied to.

https://bugzilla.gnome.org/show_bug.cgi?id=712912
Use selected text in quotes when available

https://bugzilla.gnome.org/show_bug.cgi?id=712912
Use bottom-posting when replying with selection

Forwarding always uses top-posting, since it makes it hard to
distinguish between the original mail and a bottom-posted reply.

https://bugzilla.gnome.org/show_bug.cgi?id=712912
Add selected text as quote in existing reply

This happens only if it's not changing the reply type.  This helps in
the case where you are switching the reply and haven't de-selected text
that was previously quoted.

https://bugzilla.gnome.org/show_bug.cgi?id=712912
Place cursor appropriately in new composer

When bottom posting, we need to place it between the quote and the
signature.  Rather than trying to smoosh guess about where it should go,
we mark the spot explicitly when adding (or not adding) the signature.

https://bugzilla.gnome.org/show_bug.cgi?id=712912
Preserve newlines when quoting selected plain text

https://bugzilla.gnome.org/show_bug.cgi?id=712912
Strip quote chrome from selection when replying

https://bugzilla.gnome.org/show_bug.cgi?id=712912