Explicitly convert raw data to strings

Explicitly convert raw data to strings

As strings are guaranteed to use UTF-8 in the GNOME platform, generic
file APIs like g_file_load_contents() return raw data instead. Since
gjs' recent update to mozjs60, this data is now returns as Uint8Array
which cannot simply be treated as string - its toString() method boils
down to arr.join(',') - so use gjs' new ByteArray module to explicitly
convert the data.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/179