Fix a potential bug with mouse button emulation on Windows

Re: #185.

If an error occurs when emulating mouse button events with the
SendInput() function and the primary mouse button was set by the
user to the right button, then that setting would not be restored,
leaving our left-handed user understandably confused the next time
they use the mouse.  This fixes that potential bug by swapping the
primary button back in a 'finally' block instead.

dragonfly/actions/mouse/_win32.py

@@ -108,17 +108,17 @@ class ButtonEvent(BaseButtonEvent):
 
     def execute(self, window):
         # Ensure that the primary mouse button is the *left* button before
-        # sending events.
+        #  sending events.
         primary_changed = windll.user32.SwapMouseButton(0)
-
-        # Prepare and send the mouse events.
-        zero = pointer(c_ulong(0))
-        inputs = [MouseInput(0, 0, flag[1], flag[0], 0, zero)
-                  for flag in self._flags]
-        array = make_input_array(inputs)
-        send_input_array(array)
-
-        # Swap the primary mouse button back if it was previously set to
-        # *right*.
-        if primary_changed:
-            windll.user32.SwapMouseButton(1)
+        try:
+            # Prepare and send the mouse events.
+            zero = pointer(c_ulong(0))
+            inputs = [MouseInput(0, 0, flag[1], flag[0], 0, zero)
+                      for flag in self._flags]
+            array = make_input_array(inputs)
+            send_input_array(array)
+        finally:
+            # Swap the primary mouse button back if it was previously set
+            #  to *right*.
+            if primary_changed:
+                windll.user32.SwapMouseButton(1)