Compare tab freezes when loom contains at least one annotation with a lot of unique values

[dweemx]

Problem

When a loom file contains an annotation with a lot of unique values, the Compare tab will freeze since it's trying to render a lot of DOM elements.

Solution

Limit the annotations to be used in the Compare tab e.g.: only allow annotations that have less than MAX_NUMBER_OF_UNIQUE_VALUES.

[dweemx]

See #393