[core] restructure to allow PIP packaging

OK - so I have to admit I *hate* the fact that PIP seems to require a
subdirectory named like the library.

But since the PIP package is something really nifty to have (thanks to
@tony again!!!), I updated the codebase to hopefully conform with what
PIP expects. Testruns so far look promising...

bumblebee_status/modules/core/cpu.py

@@ -0,0 +1,41 @@
+# pylint: disable=C0111,R0903
+
+"""Displays CPU utilization across all CPUs.
+
+Parameters:
+    * cpu.warning : Warning threshold in % of CPU usage (defaults to 70%)
+    * cpu.critical: Critical threshold in % of CPU usage (defaults to 80%)
+    * cpu.format  : Format string (defaults to '{:.01f}%')
+"""
+
+import psutil
+
+import core.module
+import core.widget
+import core.input
+
+
+class Module(core.module.Module):
+    def __init__(self, config, theme):
+        super().__init__(config, theme, core.widget.Widget(self.utilization))
+        self.widget().set("theme.minwidth", self._format.format(100.0 - 10e-20))
+        self._utilization = psutil.cpu_percent(percpu=False)
+        core.input.register(
+            self, button=core.input.LEFT_MOUSE, cmd="gnome-system-monitor"
+        )
+
+    @property
+    def _format(self):
+        return self.parameter("format", "{:.01f}%")
+
+    def utilization(self, _):
+        return self._format.format(self._utilization)
+
+    def update(self):
+        self._utilization = psutil.cpu_percent(percpu=False)
+
+    def state(self, _):
+        return self.threshold_state(self._utilization, 70, 80)
+
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4