Unit tests for command line arguments

This patchset includes an new python unit test for testing path passed as command line arguments. It especially tests paths with spaces and special characters (accents).
Since there is no automated GUI testing framework yet, it relies on starting a QGIS instance, waiting for some file to be created (eventually by means of a small plugin) and killing the instance after some timeout. So, some graphical parts of QGIS may be shown during the test. This is not very clean, but circumvents the lack of GUI testing for now.

It also includes fix for path parsing in main.cpp as well as encoding issues in QGIS_PLUGINPATH detected during development of these tests

Refs Redmine issue #8045