GUI: move the "mark as reply" selector to the row of buttons

There's a GCI task for changing various replying modes from within the composer
(think reply-to-all vs. reply-privately). That change needs a button to be added
somewhere, so it makes sense to use a button for the feature which Thomas added
yesterday. Besides, it seems to me that it is a bit easier to discover this way.

This is best viewed as a diff from commit
944a49e33562bd508a2af6e0dcfc938ba2769f1b; that way it is clearer what has
changed compared to the standalone-checkbox approach.

v2 changes:
- Hide the button altogether when not replying instead of just disabling it.
- Pretty formatting of the tooltips and sanitization of HTML input
- Better captions

Thanks to Thomas Lübking for his comments.

REVIEW: 114815
Importing the icons for threaded/flat view from Oxygen

Sourced from format-indent-more and format-justify-fill.
GUI: use icons for message threading