Telescope: Deal correctly with empty marks on delete and move up/down

[iserpent]

Telescope extension breaks when a mark is removed from the middle of the list.
The pull request is to add some more consistency here:

• Telescope shows only non-empty marks
• Empty marks are preserved to conform with the initial idea to keep the stable indexes
• Moving a mark up/down respects the positions of empty marks (in Telescope it appears as if the mark was not moved if it exchanges with the empty mark)

Not fixed:

• Current selection resets when the view is refreshed after the mark is moved up/down

This pull request is not directly related to the encode/decode issue, but rather complements the functionality. It doesn't depend on the encode/decode related pull requests and can be merged independently.