Account for old versions of Pygments

Account for old versions of Pygments

Pygments versions prior to 2.0 only accept lower case names for lexers.
This commit makes sure to always use lower case names for the language
that is written in the LaTeX file, while retaining the proper casing
for the presentation in the GUI, which is dictated by compatibility
with the listings package. Moreover, if one switches from listings
to minted in a document, the language combo is properly updated even
if the used language had attached a dialect (a concept not shared by
minted), or even when importing a LaTeX document with tex2lyx.