Python bindings

[mosra]

Split from https://github.com/mosra/magnum/pull/228. Things to do:

• [x] everything reasonable base from the Math lib
  • [x] testing basic functionality from python
  • [x] setting up Travis build
  • [x] setting up coverage for both C++ and Python
  • [x] GL testing on the CI
  • [x] think about the underscore policy (xaxis() might be okay but shearxz() definitely isn't) -- went back to all underscores, everything else is inconsistent
  • [x] convince Python that it can have a single method that's both static and instance (Matrix3.scaling() vs mat.scaling())
  • [x] convince Python that it can have a property and static method of the same name (Matrix3.translation() vs mat.translation) -- postponed
• [x] basic stuff from Platform
  • [x] solve protected destructors and such -- my own deleter
  • [x] how to support overriding virtual functions to subclass Platform::Application classes? -- pybind is nice
• [x] ffs, $<TARGET_EXISTS> doesn't exist until CMake 3.12, need other solution
• [x] basic stuff from GL (render a cube) in progress
  • [x] how to support variadic templates (in particular Mesh::addVertexBuffer())? *args? postponed
• [x] basic stuff from Shaders in progress
• [x] basic stuff from SceneGraph (render a cube using scenegraph)
• [x] solve inter-module dependencies (currently it's implicit and doesn't really work) it "just works" now
• [x] how to document this? mosra/m.css#98
• [x] solve compile times (I barely started and this adds 20 seconds to compile times...) -- split into some files, would need to rewrite pybind from scratch to fix this properly
• [x] how to pass big chunks of binary data through fast? --- buffer protocol, need to rewrite pybind to do this properly

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@f7d5d91). Click here to learn what that means. The diff coverage is 93.18%.

@@            Coverage Diff            @@
##             master       #1   +/-   ##
=========================================
  Coverage          ?   93.18%           
=========================================
  Files             ?       26           
  Lines             ?     1805           
  Branches          ?        0           
=========================================
  Hits              ?     1682           
  Misses            ?      123           
  Partials          ?        0

Impacted Files	Coverage Δ
src/python/corrade/PybindExtras.h	100% <100%> (ø)
src/python/magnum/__init__.py	100% <100%> (ø)
src/python/magnum/NonDestructible.h	100% <100%> (ø)
src/python/magnum/platform/windowlessapplication.h	100% <100%> (ø)
src/python/magnum/trade.cpp	100% <100%> (ø)
src/python/magnum/math.h	100% <100%> (ø)
src/python/magnum/math.matrixfloat.cpp	100% <100%> (ø)
src/python/magnum/PyMesh.h	100% <100%> (ø)
src/python/corrade/EnumOperators.h	100% <100%> (ø)
src/python/corrade/__init__.py	100% <100%> (ø)
... and 16 more

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[mosra]

Eh, I would say this is enough for the initial MVP.