Update desklet.js

I'd like to see a color-chooser included, so this is a suggested solution which works smoothly for me. Changed "desklet.js" file includes the new property (line 30) and a modified "_onSettingsChanged" function (line 42). Also requires the modified "settings-schema.json", which adds a "font-color" parameter.
Merge pull request #3428 from 3n4rch3/patch-2

Update desklet.js