Fix NextOutput and PreviousOutput when Shell::activate returns None

[jackpot51]

This fixes #160, but I am not sure when Shell::activate is supposed to return a position versus when it is not.

[jackpot51]

When using the focus shortcuts, this activates whatever window was last active on that output, not the window in the focus direction. Fixing this probably requires not falling back to next/previous output in the focus shortcuts.

[Drakulix]

Shell::activate is supposed to return a position versus when it is not.

The position is used to actually move the cursor to the new output (which can't happen inside the Shell module).
So this sounds more like an oversight to be, that Shell::activate is not returning a position, when it is clearly switching outputs and not just a workspace on the same output.

I'll merge this for the meantime though, as I am rewriting large parts of the workspace-managing code in shell/mod.rs anyway, which should also fix this issue.

Fixing this probably requires not falling back to next/previous output in the focus shortcuts.

Yes.