[core] Rework core implementation

Experimental re-implementation of core functionality with the aim:
- Depend only on the Python Standard Library for core
- If modules are missing elsewhere, *never* throw
- Unit test *everything*
- Cleaner and more minimal implementation
- Better integration points for existing implementations (charts,
  braille, etc.)
- Full backwards-compatibility with existing module system (except where
  modules can be vastly simplified)

themes/icons/test.json

@@ -1,6 +0,0 @@
-{
-	"test-widget": {
-		"prefix": "widget-prefix",
-		"suffix": "widget-suffix"
-	}
-}