UI for editing path equations in schemas and ologs

[kasbah]

Following on from #1061 this is the editing UI for equations. Closes #63 .

[kasbah]

I could use some help formulating some brief statements about this to put into the logics docs.

[epatters]

Thanks Kaspar! While this isn't a code review, I'm recording a few problems I noticed while testing.

1. Cosmetic issue: the vertical alignment of "E" in the bottom path is off:

   

2. Another cosmetic issue: the vertical sizing of "E" in the popover is off:

   

3. Logic issue: it is not currently possible to build path with repeat edges:

   

This is important because in practice a path in a path equation often uses the same generating morphism twice. In this example (the schema for symmetric graphs), I am trying to express that "inv" is indeed an involution, i.e., composing it with itself gives the identity.

[kasbah]

I changed the approach completely to allow for repetition. It now uses the list editor to form paths and is based on #1326 .