[Clean Sheet]: Check if Source Code Pro font has already been installed during bundle start

[fappel]

If Source Code Pro has already been installed and the installed version differs from the one that would be loaded by the clean sheet theme plug-in there might be font depiction errors when choosing different font styles. See #71 for more information.

We might check whether the font family has already been installed and if so consider skipping the font-loader mechanism. This means the installed font version would be preferred to the one that is shipped with Clean Sheet even if that version is older.