Terminal layout
A rudimentary layout together with relevant behaviour.
WIP: Based on !278 (merged)
WIP: Needs updated GTK: https://source.puri.sm/Librem5/gtk/merge_requests/11
EDIT: doesn't strictly need an updated GTK, this functionality is just going to stay unused without it.
Solves #126 (closed)
WIP: (still) Missing design.