Add key-colors to appstream for gnome-software

gnome-software uses some heuristics to extract ‘key’ colours from the icon of
an application. Unfortunately, those heuristics can’t always be perfect, and
for KTurtle they choose quite a dark colour. Help them out by explicitly
specifying a suitable ‘key’ colour from the icon in the app’s appdata.

The colours have been picked arbitrarily from the green area inside the
turtle icon. They have been chosen to work well when used as a block
background colour behind the icon.

See https://gitlab.gnome.org/GNOME/gnome-software/-/blob/main/lib/gs-app.c#L4239-L4248
or https://blogs.gnome.org/sophieh/2021/08/15/apps-for-gnome-on-its-mark/#LC2