Add Obstacle toggle button to StoryItemOptions

[birbilis]

Version: 0.5.12

Should add an "Obstacle" toggle button to StoryItemOptions (and respective property to IStoryItem/TStoryItem).

When dragging an unanchored child StoryItem of the ActiveStoryItem in non-Edit mode, other siblings of it that are have Obstacle=True should block its movement, unless they share the same Tag with the dragged StoryItem (in which case the Obstacle works like a Gate that filters the items that can pass-through)