[enhancement] Show locking state

[Airblader]

See #35. i3lock should display "locking..." while it is trying to grab the pointer so the user knows when the locking was successful.

[stapelberg]

Note that we should only start displaying that text after a little delay, maybe 250ms, so that in regular operations, people won’t see it (only if the operation is about to fail).

[stapelberg]

Also note how we need to pay attention to forking: we should only fork when we could grab the pointer/keyboard, and the exit code still needs to be 0/1 according to success/failure of grabbing.

…otherwise, we might break the security of people’s suspend mechanism, i.e. people who use i3lock && pm-suspend would end up with a non-locked screen.