Landscape

Solves https://source.puri.sm/Librem5/squeekboard/issues/74

Needs https://source.puri.sm/Librem5/squeekboard/merge_requests/221

TODO:

• accept Wayland output changes
• alter layer surface geometry based on output width
• add landscape layouts to layout loading system
• implement a us-landscape layout

Not TODO, unless requested:

• alter layer surface geometry based on output shape (no design)
• be smart about selecting current output (no need yet)
• use slightly altered portrait layout if landscape missing (no design, no complaints yet)