[themes] Add "cycle" theme capability

It is now possible to add a list of theme configurations in the
"default" section called "cycle". These configuration items will be
cycled through module by module. to create "alternate style" effects.
This is *only* possible in the "default" configuration part, but any
module-specific configurations still take precedence.

Also, removed the capability of per-widget themes. That simply
complicates things and probably doesn't really bring any benefits.

bumblebee/themes/default.json

@@ -1,7 +1,15 @@
 {
 	"defaults": {
 		"prefix": " ",
-		"suffix" : " "
+		"suffix" : " ",
+		"cycle": [
+			{
+				"fg": "red"
+			},
+			{
+				"fg": "lime"
+			}
+		]
 	},
 	"battery": {
 		"prefix": "bat ",