Is it common for a lot of the source code live in header files? E.g. https://github.com/aardappel/treesheets/blob/master/src/cell... I'm not being sarcastic, just curious if this is normally done these days.

It's not common. This is the inspiration: http://strlen.com/java-style-classes-in-c

Thanks for that, good read.

I hope it'll boost the development of this awesome tool. Android port anyone?