Events should link to WordClusters, not tags

[herongrove]

Current behavior

In the parsers, single-token arguments of events are represented by token tags (in the default layer of tags), while multi-token arguments are represented by WordClusters. See for example this if/else statement. This has unfortunate effects, such as:

1. changing the display to hide the default layer will not remove the WordClusters, which are managed separately
2. deleting a WordCluster that is linked to should also delete the link to that WordCluster, whereas deleting a label doesn't clearly indicate that the link to the token index should be deleted
3. every argument should be editable to change its span (e.g. to include an additional token), which is more natural to accomplish if it is already a WordCluster

Intended behavior

All TextboundMentions should be represented as WordClusters. For example, in the Odin parser linked above, only the lines within the else block should be used. Similar behavior should be exhibited by the other parsers.