I watched WWDC 2016 video 414 and came to know that I can write my own editor extension. The very first thing that comes to my mind is "bookmark"! Not sure why Xcode does not have this feature.
I read through Xcode docs about XCSourceEditorExtension/XCSourceEditorCommand, but could not find any docs about how to manipulate editor gutter so that I can add bookmark images using code. Or is this just not possible?