Add return type for constructor functions

[scemino]

Today it's impossible to use all constructor functions, I modified the generator to fix this. Actually I needed to use ImGuiTextFilter to filter a list and I found no way to do this with the actual nimgl version. I hope you'll find it useful.

[scemino]

I found a workaround, it is possible to use ImGuiTextFilter like this: filter.addr.draw("Filter (\"incl,-excl\") (\"error\")", 180), I hope that it will help other people which have the same problem.