Add Unvisited/Visited StoryPoint toggle to show only visited ones in StructureView

[birbilis]

At
#328
removed code that was looping back to end of story when Previous StoryPoint action was done

User can still cheat and jump to end of story only via the StructureView

Can add a "Visited" StoryPoint state apart from the "Unvisited" StoryPoint state (e.g. via an extra Visited property) to StoryPoints to allow showing only previously visited ones (or ones the author set as such from beforehand)

Make StoryPoint options button as three-state one: non-StoryPoint, unvisited StoryPoint, visited StoryPoint

Internally should add a "Visited" boolean property since there was a "StoryPoint" boolean property already. The "Visited" property should have default value to false (e.g. if missing from older stored state).

Probably useful anyway in the future to have such state e.g. for marking StoryPoints one visited on a map (show them with some glow for example) or have them locked (if not visited from other StoryPoints to get unlocked) etc.

[birbilis]

obviously (not-StoryPoint, Visited) combination has no meaning, that's why it would be a three-state button:

• non-StoryPoint non-Visited
• StoryPoint non-Visited
• StoryPoint Visited

setting StoryPoint=false should also set Visited=false
setting Visited=true should also set StoryPoint=true