I should look at that more closely next time I'm on. It looks pretty much identical to my font. Maybe it's that it's identical to the default font. That does make sense though. The current one just seemed a bit too small for me. That's great to know you're wanting to add things like that to the gutter. ProJucer definitely has the potential to be an awesome development environment.
Edit: Yeah, Visual Studio's line numbers do actually match the font. Sublime Text has the same behavior. Okay, so Atom has this behavior too. It looks like this is the standard way of doing it, at least from the editors i use and love. I'll have to try others.
If the ProJucer does go that route, one thing I (and other Vim users) won't be able to live without is some kind of Vim mode. That's my main concern with the project I'm working on, actually. There's a lot of awesome open source Vim emulation plugins out there to get ideas from, so hopefully I can create something useful. I actually doing even use Vim because it's just too constraining in ways, but can't live without the plugins in the editors I use.