Make palettes lazy

[erikos]

We want to make palettes lazy to reduce memory usage.

If the palette needs to be updated (e.g. the palette of an Access POint or Ad-hoc network) we add the logic in a model class that both the icon and the palette listen to and propagating the NM signal with gobject signals or have the icon and palette listening directly to NM, depends how much code is that and how tricky.

[dnarvaez]

Not clear if this refers to specific palettes. I don't think we should do it for every palette because of the increased code complexity.