[core/themes] Add state themes

Each widget can now return a state using the method "state()". This
string is then used to look up a theme information which is used instead
of the default or module theme, if found.

see #23

themes/test.json

@@ -6,10 +6,17 @@
 		"fg": "#000000",
 		"bg": "#111111",
 		"separator": " * ",
-		"separator-block-width": 10
+		"separator-block-width": 10,
+		"critical": {
+			"fg": "#ffffff",
+			"bg": "#010101"
+		}
 	},
 	"test-widget": {
 		"fg": "#ababab",
-		"bg": "#222222"
+		"bg": "#222222",
+		"critical": {
+			"fg": "#bababa"
+		}
 	}
 }