Display angle structure matching equations in the GUI

[baburton]

For extra points, also include the peripheral equations for meridians and longitudes (in the case of a SnapPeaTriangulation).

[baburton]

Note: the peripheral equations are the ones computed by gluing_equations() from SnapPy.

[S-Rafael]

I may or may not have been one of the users who requested this feature...
So here is my shot at it:

Is that how you would like the ouput to look? Right now, it's a read-only text area which allows the user to easily select and copy out the matrix as a string (optionally, formatted as Latex source).

I haven't done the peripheral equations yet: it's not as simple as calling regina::snappea::get_cusp_equation() because Snappea seems to often simplify the triangulation (via Pachner 3-2 moves) before doing anything else, so we would be getting the equations for the wrong triangulation.

Any feedback will be appreciated, and if you like this feature, I can submit a pull request.