[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/test_cycle.json

@@ -1,18 +0,0 @@
-{
-	"icons": [ "test" ],
-	"defaults": {
-		"prefix": "default-prefix",
-		"suffix": "default-suffix",
-		"fg": "#000000",
-		"bg": "#111111"
-	},
-	"cycle": [
-		{ "fg": "#aa0000" },
-		{ "fg": "#00aa00" },
-		{ "fg": "#0000aa" }
-	],
-	"test-widget": {
-		"fg": "#ababab",
-		"bg": "#222222"
-	}
-}