EncoderLaTeX: improving math mode support

Improving tracking of LaTeX's inline math mode, i.e. mathematical
expressions inside dollar symbols ($\alpha$) and inside the ensuremath
command (\ensuremath{\alpha}).
This change required introducing a stack tracking which mathematical
environments are currently active (as they may be nested) as well as
opening and closing curly brackets to recognize when an ensuremath
command ends.

A new test in KBibTeXIOTest got added to ensure that inline mathematical
expressions get correctly encoded and decoded.