WIP - Pre-select works in single-select mode and moves user selection too

[vitalybe]

This is related to my need here - I want pre-selection to move the actual initial user selector and work also in single mode (without the -m flag).

At this point it works, even though the code is very hacky - Mainly, I am using the EvHeartBeat event to check whether the cursor can be moved or not yet.

I would love to hear your feedback:

1. As a feature - Is it something you'd like to merge into the project?
2. Do you have a better idea than putting the callback in EvHeartBeat?
3. What else should be done to merge this feature into skim?