WIP: Adjust page content if no touchpad is present

Not all systems have touchpads. Show keyboard instructions for triggering the overview and switching workspaces if no touchpad is present.

For now we assume that all touchpads support three-finger swipes.

This has a number of extremely big caveats:

1. I was not able to (quickly) work out how to link to gnome-settings-daemon (a boring C pkg-config library) and call two C functions, passing the result of one plus a constant to the other, and comparing the result to NULL. There is a comment indicating the equivalent C code which is needed. Perhaps a seasoned Rust expert would be able to fill this in relatively easily In the meantime (and for future testing), TOUR_FORCE_TOUCHPAD=1 will cause the touchpad path to always be taken, and TOUR_FORCE_TOUCHPAD=0 will cause the non-touchpad path to be taken. Hopefully this unblocks the strings & artwork.
2. As written, the strings on the pages that would normally be about touchpads are placeholders, and there is no artwork. I see that the animations are implemented in CSS, so I added a CSS class to the parent widget of .touchpad or .no-touchpad which can be used to distinguish in the CSS.

Closes #27