[core] Refactoring to use an event dispatcher in the main thread.

Plugins now run in the main thread, instead of their own threads, and
the API changed to use an event dispatcher approach instead of a static
class interface.

Projectfile

@@ -42,11 +42,12 @@ python.setup(
 python.add_requirements(
     'fs >=2.0,<2.1',
     'jinja2 >=2.9,<2.10',
-    'mondrian ==0.4a0',
+    'mondrian ==0.4a1',
     'packaging >=16,<17',
     'psutil >=5.4,<6.0',
     'requests >=2.0,<3.0',
     'stevedore >=1.27,<1.28',
+    'whistle ==1.0a3',
     dev=[
         'pytest-sugar >=0.8,<0.9',
         'pytest-timeout >=1,<2',