[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/iceberg.json

@@ -1,59 +0,0 @@
-{
-    "icons": [ "awesome-fonts" ],
-    "defaults": {
-        "separator-block-width": 0,
-        "separator": "",
-        "warning": {
-            "fg": "#0f1117",
-            "bg": "#e2a478"
-        },
-        "critical": {
-            "fg": "#0f1117",
-            "bg": "#e27878"
-        },
-        "fg": "#c6c8d1", "bg": "#161821"
-    },
-    "dnf": {
-        "good": {
-            "fg": "#0f1117",
-            "bg": "#b4be82"
-        }
-    },
-    "apt": {
-        "good": {
-            "fg": "#0f1117",
-            "bg": "#b4be82"
-        }
-    },
-    "pacman": {
-        "good": {
-            "fg": "#0f1117",
-            "bg": "#b4be82"
-        }
-    },
-    "battery": {
-        "charged": {
-            "fg": "#b4be82",
-            "bg": "#161821"
-        },
-        "charging": {
-            "fg": "#89b8c2",
-            "bg": "#161821"
-        }
-    },
-    "pomodoro": {
-        "paused": {
-            "fg": "#0f1117",
-            "bg": "#e2a478"
-        },
-        "work": {
-            "fg": "#1d2021",
-            "bg": "#b8bb26"
-        },
-        "break": {
-            "fg": "#89b8c2",
-            "bg": "#161821"
-        }
-    }
-
-}