Improve CPU usage in Hints mode

Improve CPU usage in Hints mode

Raising the "check for marker element movements" interval from 0 to 15 results
in no noticeable CPU usage change on my system, while still allowing markers to
move smoothly. Setting it to 20 results in noticeable stutter.

This commit also add the `hints_sleep` pref to control the interval, or disable
the feature altogether.

Additionally, the interval now doesn't run at all for the `zF` command.
Previously, it ran as usual, but didn't do anything (as marker movement isn't
supported (YAGNI) for the `zF` command).

Fixes #678.