Remove label_font in the constructor of Button.

For the sake that it will not clutter up the code. Made as a nice-to-have at #39.

components/common_ui.py

@@ -57,7 +57,7 @@ class Button(CommonUI):
     VPADDING = 18
     HPADDING = 18
 
-    def __init__(self, label, color, position, label_font=None):
+    def __init__(self, label, color, position):
         """
         Draws a rectangular button.