[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

bumblebee/modules/layout.py

@@ -1,67 +0,0 @@
-import subprocess
-import shlex
-import bumblebee.module
-import bumblebee.util
-
-def description():
-    return "Showws current keyboard layout and change it on click."
-
-def parameters():
-    return  [
-        "layout.lang: pipe-separated list of languages to cycle through (e.g. us|rs|de). Default: en"
-    ]
-
-
-class Module(bumblebee.module.Module):
-    def __init__(self, output, config):
-        super(Module, self).__init__(output, config)
-
-        self._languages = self._config.parameter("lang", "en").split("|")
-        self._idx = 0
-
-        output.add_callback(module=self.instance(), button=1, cmd=self.next_keymap)
-        output.add_callback(module=self.instance(), button=3, cmd=self.prev_keymap)
-
-    def next_keymap(self, event, widget):
-        self._idx = self._idx + 1 if self._idx < len(self._languages) - 1 else 0
-        self.set_keymap()
-
-    def prev_keymap(self, event, widget):
-        self._idx = self._idx - 1 if self._idx > 0 else len(self._languages) - 1
-        self.set_keymap()
-
-    def set_keymap(self):
-        tmp = self._languages[self._idx].split(":")
-        layout = tmp[0]
-        variant = ""
-        if len(tmp) > 1:
-            variant = "-variant {}".format(tmp[1])
-        bumblebee.util.execute("setxkbmap -layout {} {}".format(layout, variant))
-
-    def widgets(self):
-        res = bumblebee.util.execute("setxkbmap -query")
-        layout = None
-        variant = None
-        for line in res.split("\n"):
-            if not line:
-                continue
-            if "layout" in line:
-                layout = line.split(":")[1].strip()
-            if "variant" in line:
-                variant = line.split(":")[1].strip()
-        if variant:
-            layout += ":" + variant
-
-        lang = self._languages[self._idx]
-
-        if lang != layout:
-            if layout in self._languages:
-                self._idx = self._languages.index(layout)
-            else:
-                self._languages.append(layout)
-                self._idx = len(self._languages) - 1
-        lang = layout
-
-        return bumblebee.output.Widget(self, lang)
-
-# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4