Annotate equation/variable kind in structure view

[Keno]

The mss display should gain two additional columns for variable/equation kind. This may need some interface changes in StateSelection.jl.

For example, in the following:

The equation pairs on the states are actually initial equations, but this confused me when I first looked at it.

[Keno]

While we're at it, we should also annotate which equations are fully linear, since those get treated specially.

[Keno]

I've done some of this in #43, but we should still add columns for which callee the equation or variable is in

[serenity4]

Here is what I got so far:

Does that roughly look like what you had in mind? (will put up the PR later after some cleanup)

[Keno]

Yep, looks good