Reduce load on structure viewer when moving mouse over ruler

JAL-4086 introduced highlights for all positions aligned at a particular column. When lots of structures are displayed these highlights may be difficult to interpret, and can slow down Jalview and the linked structure viewer - even more so with JalviewJS.

Ideally, mouse overs should not block other operations on the alignment view, and the structure viewer should remain responsive. Rather than emitting a highlight command for each moused-over column, the viewer should use an observer pattern to reflight highlights in a non blocking way, allowing the use of a timer in JalviewJS.