composer: Adaptive toolbar

Splits the toolbar at the top of the composer into two action bars at the bottom. One is always visible and contains undo, redo, spellcheck, a menu, and a button to toggle the other action bar. The other action bar contains the formatting buttons.

Also cleans up the margins around the fields at the top.

Implemented according to the mockups at https://gitlab.gnome.org/Teams/Design/app-mockups/raw/master/mail/composer.png