Add support to load resources from a config file

A new file called resources.cfg has been created in order
to indicate resources.
Remove defined-variable OGRE_PLUGIN_VAR

Plugin directory is now indicated in plugins.cfg file
Clean up example.pro and lib.pro files

There was left two unused variables in the unix-related build part of the project files
example.pro and lib.pro, namely OGRELIBDIR and OGREPLUGINDIR. Besides of that, two install
targets have been added in example.pro in order to copy resources and config files to
the build directory.