Use multiple seats for keyboards with different layouts

[emersion]

Create one seat per keyboard group. Sync all of the seats' keyboard focus.

This prevents a bunch of issues. For instance assuming keyboards A and B have a different layout: pressing a key on keyboard A, pressing another key on keyboard B, and releasing a key on keyboard A results in messed up keyboard state.

[emersion]

Maybe something like seat <name> follow_focus <other> would be useful.