[core] Add input handling

Add a (half-finished) input library, that for now simply allows
registration and triggering of events.

As next steps, the trigger will happen as part of a separate thread that
reads input events.

Additionally, invoking commands via a execute() will be supported.

Thirdly, there is need of a way to selectively update the affected
modules (widgets), which should be possible given that the event
contains both the instance (widget ID) and name (module name).

modules/datetime.py

@@ -13,13 +13,13 @@ import locale
 
 import core.module
 import core.widget
+import core.input
 
 class Module(core.module.Module):
     def __init__(self, config):
         super().__init__(config, core.widget.Widget(self.full_text))
-        # TODO: register callback
-        #engine.input.register_callback(self, button=bumblebee.input.LEFT_MOUSE,
-        #                            cmd='calendar')
+
+        core.input.register(self, button=core.input.LEFT_MOUSE, cmd='calendar')
         self._fmt = self.parameter('format', self.default_format())
         l = locale.getdefaultlocale()
         if not l or l == (None, None):