Improve the command-line parser meant to be used for QProcess.

Improve the command-line parser meant to be used for QProcess.
Now, both single- and double-quotes posix-like quoting styles are implemented,
such that a command-line meant for the shell can also be used for QProcess.
However, only redirecting the output to a file is supported. Supporting piping
would require a (major?) rewrite of the Systemcall class.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@32590 a592a061-630c-0410-9148-cb99ea01b6c8