Use pylint to make xtextcontent.py more pythonic. Mostly,
changing variable and method names to comply with the
python style guide. Also, remove unneeded imports.

Change-Id: I80e6fa53e67a86520a85284f3dad76a614450047
Reviewed-on: https://gerrit.libreoffice.org/66199
Tested-by: Jenkins
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>