Use gchar* instead of GString

The IMAP scanner returns instances of GString. This is not really
necessary since we don't use anything that GString offers. Also, the
list scanner returns an array of gchar*, and using the same type for
atoms would be nice for consistency.

Make libmx_imap_scanner_scan_atom return gchar* instead of GString.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>