ESourceRegistry: Ignore hidden files from .source folder monitoring

There used to be a critical warning on the source registry console:
   ** Failed to load key file at
   'file:///home/user/.config/evolution/sources/.goutputstream-1P9V6W':
   File must have a '.source' extension

There is nothing the source registry can do with these files (because
they are created by GIO), but ignore them during the folder monitoring,
instead of spreading useless runtime warning.