[feature-request] Enable stepping through time not just by more than just a time increment

[Keith Hughitt]

User Request:

Enable selection of a criterion other than an absolute time increment to adjust time by when the "next" or "previous" time buttons are pressed. Some ways the users may want to navigate by:

1. Time of next/previous image for a specified image layer
2. Time of next/previous event for a specified event layer
3. Time of next/previous image for ANY image layer
4. Time of next/previous event for ANY event layer
5. Time of next/previous image/event for ANY event OR image layer.

One of the goals of the above is to eliminate clicks where nothing changes: Instead, for every step through time you are guarantied that at least something will change.

A "control" layer could possibly be used to specify a specify layer which the time navigation should follow.