Refactoring to remove global state

[publixsubfan]

This PR is intended to split off some of the work from #149 into a somewhat-smaller PR.

• Adds a new class, GLVisWindow, which consolidates most of the global state within aux_vis.cpp. GLVisWindow handles creation of the SdlWindow, VisualizationScene*, and GLVisCommand objects from a StreamState object and/or an array of input streams.
• Make VisualizationScene* event handlers member functions where possible, removing dependence on global pointers.
• Remove most references to global objects from the core glvis library.
• Move handling of multiplexing of GLVisCommand and other idle function runs from SdlWindow to GLVisWindow.

[tomstitt]

(minor) these could be default values when the members are declared:

... etc ...
bool face_has_kerning = false;
bool is_hidpi = false;
int ppi_w = 96, ppi_h = 96;

[tomstitt]

I can now build and run the js version but there is at least on thing that isn't working yet. I saw that the CallKeySequence call was moved from aux_js.cpp:startVisualization into aux_vis.cpp:InitVisualization (which is called in aux_js.cpp:startVisualization) but it doesn't seem to work, when I open ex9 I see this:

Instead of this:

I'll spend some more time debugging.

[tomstitt]

The JS target seems to work well for me now, I slacked a copy to @tzanio and @kanye-quest so check it out if you want

[tzanio]

All other examples from glvis.org/live work for me, but I still get an error with Navier in this branch, e.g. the following run:

./glvis -saved ~/glvis-web/src/data/navier.saved

shows

instead of

[tzanio]

Preliminary review

[tzanio]

I think this may be the first use of exceptions in MFEM or GLVis 😁

[tzanio]

one empty line