launcher-logind: Allow CTRL+ALT+Fx to work on seats with no TTYs
The launcher-logind backend when this keyboard command is invoked calls logind's SwitchTo method, which works on seats with no TTYs CTRL+ALT+F1 switches to the first logind session on the seat, as CTRL+ALT+F2 switchis to the second logind session on the seat, and so on.