Try to use the default font from mpl

[vallsv]

Closes #4018

The DejaVu font is packaged together with matplotlib. Better to take advantage of that font than expecting the user environment to provides font with every expected characters.

The opengl backend use a qfont and then try to get back a matplotlib font for the axes. This was not properly working.
So i have more or less forced a sans-serif font. That anyway the default for us, for readability.

Changelog:

• Use matplotlib font for OpenGL backend

[vallsv]

Notice that the axes labels use the rasterMathText which is already a mpl backend, which is maybe way there is still a problem.

[vallsv]

Also the code import rasterMathText as optional, but this API is now mandatory

[vallsv]

The real fix the prepend of the sans-serif in the QFont->mpl font

[t20100]

LGTM

In the mid-term we should think of how to make both backends more compatible: It's probably worth upgrading this and handle all those kind of defaults once for both backends.