[engine/popup] Make tkinter module optional

If tkinter is not available, status bar should still work as expected
(but all the popup functionality will not work and likely throw errors).

bumblebee-status

@@ -42,6 +42,7 @@ def main():
         inp.stop()
         sys.exit(0)
     except BaseException as e:
+        if not engine: raise
         module = engine.current_module()
         logging.exception(e)
         if output.started():

bumblebee/popup.py

@@ -4,7 +4,10 @@ try:
     import Tkinter as tk
 except ImportError:
     # python 3
+    try:
         import tkinter as tk
+    except ImportError:
+        pass
 
 import logging