Main executable: removed dependencies on some of the Core plugin features (closes #508).

I initially wanted to keep common.cpp/.h for the main executable, but
that didn’t work for the CLI version of OpenCOR on Windows since
common.cpp/.h contains both CLI and GUI utilities. So, now, we have
cliutils.cpp/.h and guiutils.cpp/.h rather than common.cpp/.h, which
means that the former cliutils.cpp/.h and guiutils.cpp/.h are now
corecliutils.cpp/.h and coreguiutils.cpp/.h.