WIP Refactored the axes module to split it in smaller chunks.

This PR is work in progress.

I've started refactoring the axes module to split it into several files. In this PR, I've split the axes 9kloc module, into 3 submodules:

   - _axes, which is "only" 6kloc, containing all the code linked to plots
   - _base (3kloc), which contains mostly a base class for Axes to inherit; It contains everything linked to drawing, interactive manipulation, data limits & ticks formatting, adding & tracking artists
   - _subplots, which contains the classes and functions about subplots.

This may not be the best way to split the axes module: feedback is welcome.

All tests pass (I had to modify some). These changes are completely backward compatible, except for private methods, that are not accessible from the axes module anymore.