[core/themes] Add module-specific themes

Allow module-specific theme information to overload "default"
configuration. I.e. it is now possible to have specific prefix or
postfix configurations for different modules. The module name is derived
for each widget from the module (__module__) from which it was
instantiated.

see #23

themes/icons/awesome-fonts.json

@@ -1,5 +1,5 @@
 {
-	"defaults": { "separator": "" },
+	"defaults": { "separator": "", "padding": " " },
 	"date": { "prefix": ""	},
 	"time": { "prefix": "" },
 	"memory": { "prefix": "" },