[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/gruvbox-powerline.json

@@ -1,72 +0,0 @@
-{
-	"icons": [ "paxy97", "awesome-fonts" ],
-	"defaults": {
-		"warning": {
-			"fg": "#1d2021",
-			"bg": "#d79921"
-		},
-		"critical": {
-			"fg": "#fbf1c7",
-			"bg": "#cc241d"
-		},
-		"default-separators": false,
-		"separator-block-width": 0
-	},
-	"cycle": [
-		{
-			"fg": "#ebdbb2",
-			"bg": "#1d2021"
-		},
-		{
-			"fg": "#fbf1c7",
-			"bg": "#282828"
-		}
-	],
-	"dnf": {
-		"good": {
-			"fg": "#002b36",
-			"bg": "#859900"
-		}
-	},
-	"apt": {
-		"good": {
-			"fg": "#002b36",
-			"bg": "#859900"
-		}
-	},
-	"battery": {
-		"charged": {
-			"fg": "#1d2021",
-			"bg": "#b8bb26"
-		},
-		"AC": {
-			"fg": "#1d2021",
-			"bg": "#b8bb26"
-		}
-	},
-  "bluetooth": {
-      "ON": {
-			    "fg": "#1d2021",
-			    "bg": "#b8bb26"
-      }
-  },
-  "git": {
-	"modified": { "bg": "#458588" },
-	"deleted": { "bg": "#9d0006" },
-	"new": { "bg": "#b16286" }
-  },
-    "pomodoro": {
-        "paused": {
-            "fg": "#1d2021",
-            "bg": "#d79921"
-        },
-        "work": {
-            "fg": "#1d2021",
-            "bg": "#b8bb26"
-        },
-        "break": {
-            "fg": "#002b36",
-            "bg": "#859900"
-        }
-    }
-}