make compatible with OpenGL 3.0 / GLSL 1.30

now it runs on mesa.
The catalyst driver broke on one of my dev machines so I have
to use the open source driver now
AMD needs to get their act together.
extend cursor eqn text, limit cursor disp precision
replace all Gtk::Stock buttons to free Alt+A

I'm getting sick of finding these sneaky
shortcut keys
add default dirs, filenames for save/load
add zoom keys (x,z)
add orbiting camera
apply graphs when loading them
bugfix: fix load not using prev dir
move window-level settings to menu and toolbar
add a directional light
rearrange UI elements - hopefully cleaner
add spacing between widgets
add openGL version check
change calls to exit into Gtk::Application::quit

this way destructors are called, and resources are properly freed
change _gl_window from ptr to ref
cleanup, change pointers to refs when possible
add icon
simplify icon loading
fix directional light - no longer moves w/ camera
fix segfault on invalid toolbar / menu XML