[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.json

@@ -1,25 +0,0 @@
-{
-	"icons": [ "test" ],
-	"defaults": {
-		"prefix": "default-prefix",
-		"suffix": "default-suffix",
-		"fg": "#000000",
-		"bg": "#111111",
-		"separator": " * ",
-		"separator-block-width": 10,
-		"critical": {
-			"fg": "#ffffff",
-			"bg": "#010101"
-		}
-	},
-	"test-widget": {
-		"fg": "#ababab",
-		"bg": "#222222",
-		"critical": {
-			"fg": "#bababa"
-		},
-		"cycle-test": {
-			"fg": [ "#000000", "#111111" ]
-		}
-	}
-}