Allow custom merge/replace semantics for TextPresentation ranges eclipse-platform#2543

TextPresentation performs at least two tasks when applying new
StyleRanges:
- identify which style ranges need creating/matching with new styles
- merge or replace an individual style range

Decouple the two tasks by exposing methods for applying style range(s) that
accept a BiConsumer to customise how style ranges should be merged

Author: gregdyke