Multiple bookmarks at a point location hides all but the last

Since bookmarks are point based and do not replace any text, they cannot technically overlap eachother and there's no reason multiple can't be shown at a point location. The current implementation of insertLineContent prevents them from being shown by assuming there's only ever one. This is another naive solution that has one problem: when there are two (or more) bookmarks the cursor is positioned in between them (as opposed to before all of them) when the insertLeft option is set true.