Handle "uncomplete" profiles (for gnome-terminal >= 3.8)

[aruhier]

Profiles with no visible-name key in dconf are shown as "invalid" by the script. Not a big deal, except it's not compatible with the -p | --profile.
The default profile is not created correctly by gnome (I mean differently than a profile you would create manually in gterminal), so the key default is not created in /org/gnome/terminal/legacy/profiles:. It's a problem with the scripts set_dark and set_light, which are looking for the default profile indicated by this dconf key.

Need to be fixed