ENH: sort the examples from shortest to longest
ENH: sort the examples from shortest to longest

In a gallery, we want the simplest examples in the beginning. A good
proxy for simplicity is the amount of code.

This commit adds sorting by the amount of code.

sphinx-gallery/sphinx-gallery,Eric89GXL/sphinx-gallery,Titan-C/sphinx-gallery,Eric89GXL/sphinx-gallery,lesteve/sphinx-gallery,Titan-C/sphinx-gallery,sphinx-gallery/sphinx-gallery,lesteve/sphinx-gallery