[core/theme] Add support for icon themes

Allow sub-themes ("iconsets") to be merged into the "main" theme. That
way, effectively, it's possible to define colors and icons in separate
JSON files.

see #23

themes/test.json

@@ -0,0 +1,7 @@
+{
+	"icons": [ "test" ],
+	"defaults": {
+		"prefix": "default-prefix",
+		"suffix": "default-suffix"
+	}
+}