WIP: use standard text styles

Adjust the shell to use the standard text styles that have been developed in gtk#1808 (closed) .

This branch is not ready to be merged. Various issues still exist, and the MR is just intended to facilitate review and development.

Outstanding issues:

• Since the text size increased, the calendar grid has grown in size and needs to be shrunk back down (ideally keeping the new text sizes)
• Body text in notification banners has been reduced to 9pt. To my eyes this feels a bit small, but I'm not sure what the best thing to do is, and it's what I've got in my latest mockups.
• If we keep the body text at 9pt, we need to fix a bug where it becomes 11pt on hover.
• The large date above the calendar ellipsizes when the month name is long.
• The title of mount password dialogs ellipsize: image (a shorter title would be better - "Password Required" - but we should also ellipsize the title)
• Dialogs yet to be tested:
  • Volume is busy
  • Portal request (for example, camera or microphone access)
  • Location services request
  • Keyring unlock
  • Inhibit shortcuts
  • Unresponsive app

Issues that are in bold are beyond my technical abilities! I'm also missing a way to trigger the untested dialogs.