Add Prev/Next Exercise button and hotkeys

[Voileexperiments]

It's more helpful to navigate between exercises by going forward/backward instead of just warping into the first/last unsolved exercise. Warping into the first unsolved exercise becomes unhelpful to improving workflow if there is a dangling unsolved exercise behind, as pressing n no longer warps to the next exercise to which I've just solved (which is what I would usually want), but always that dangling exercise.

Suggested hotkeys: Ctrl+Left/Right

[teorth]

This has now been implemented. I used < and > for the hotkeys.