Navigate to next/previous/first/last board position that contains <criteria>

[herzbube]

Currently the board position navigation buttons jump to the next/previous/first/last board position unconditionally. Instead the user should be able to navigate based on some "filter" or "condition".

Filter/condition ideas:

• Board position designated as a hotspot
• Board position that contains a short or long description
• Board position that contains any kind of annotation
• Board position that contains any kind of markup
• Board position that is a variation branching point

The way how this is supposed to look like in the UI is not yet clear.

Difficulty: Look back/ahead from the current board position if there is a board position that satisfies the filter/condition so that the navigation buttons can be enabled/disabled accordingly.

Alternative: Only display board positions that match the filter/condition. Navigation only applies to the filtered moves.