[core] Refactor engine

This is going to be a bit more comprehensive than anticipated. In order
to cleanly refactor the core and the engine, basically start from
scratch with the implementation.

Goals:

* Test coverage
* Maintain backwards compatibility with module interface as much as
  possible (but still make modules easier to code)
* Simplicity

see #23

themes/icons/awesome-fonts.json

@@ -1,134 +0,0 @@
-{
-	"defaults": {
-		"separator": ""
-	},
-	"date": {
-		"prefix": ""
-	},
-	"time": {
-		"prefix": ""
-	},
-	"memory": {
-		"prefix": ""
-	},
-	"cpu": {
-		"prefix": ""
-	},
-	"disk": {
-		"prefix": ""
-	},
-	"dnf": {
-		"prefix": ""
-	},
-	"brightness": {
-		"prefix": ""
-	},
-	"cmus": {
-		"states": {
-			"playing": {
-				"prefix": ""
-			},
-			"paused": {
-				"prefix": ""
-			},
-			"stopped": {
-				"prefix": ""
-			}
-		},
-		"prev": {
-			"prefix": ""
-		},
-		"next": {
-			"prefix": ""
-		},
-		"shuffle": {
-			"states": { "on": { "prefix": "" }, "off": { "prefix": "" } }
-		},
-		"repeat": {
-			"states": { "on": { "prefix": "" }, "off": { "prefix": "" } }
-		}
-	},
-	"pasink": {
-		"states": {
-			"muted": {
-				"prefix": ""
-			},
-			"unmuted": {
-				"prefix": ""
-			}
-		}
-	},
-	"pasource": {
-		"states": {
-			"muted": {
-				"prefix": ""
-			},
-			"unmuted": {
-				"prefix": ""
-			}
-		}
-	},
-	"nic": {
-		"states": {
-			"wireless-up": {
-				"prefix": ""
-			},
-			"wireless-down": {
-				"prefix": ""
-			},
-			"wired-up": {
-				"prefix": ""
-			},
-			"wired-down": {
-				"prefix": ""
-			},
-			"tunnel-up": {
-				"prefix": ""
-			},
-			"tunnel-down": {
-				"prefix": ""
-			}
-		}
-	},
-	"battery": {
-		"states": {
-			"charged": {
-				"prefix": "",
-				"suffix": ""
-			},
-			"AC": {
-				"suffix": ""
-			},
-			"charging": {
-				"prefix": [ "", "", "", "", "" ],
-				"suffix": ""
-			},
-			"discharging-10": {
-				"prefix": "",
-				"suffix": ""
-			},
-			"discharging-25": {
-				"prefix": "",
-				"suffix": ""
-			},
-			"discharging-50": {
-				"prefix": "",
-				"suffix": ""
-			},
-			"discharging-80": {
-				"prefix": "",
-				"suffix": ""
-			},
-			"discharging-100": {
-				"prefix": "",
-				"suffix": ""
-			}
-		}
-	},
-	"caffeine": {
-		"states": { "activated": {"prefix": " " }, "deactivated": { "prefix": " " } }
-	},
-	"xrandr": {
-		"states": { "on": { "prefix": " "}, "off": { "prefix": " "} }
-	}
-}