Bug: Qt mouse button MouseButton.MiddleButton is unknown

[tlambert03]

│ /Users/talley/dev/self/ndv/src/ndv/views/_qt/_app.py:107 in mouse_btn        │
│                                                                              │
│   104 │   │   if btn == Qt.MouseButton.NoButton:                             │
│   105 │   │   │   return MouseButton.NONE                                    │
│   106 │   │                                                                  │
│ ❱ 107 │   │   raise Exception(f"Qt mouse button {btn} is unknown")           │
│   108 │                                                                      │
│   109 │   def set_cursor(self, type: CursorType) -> None:                    │
│   110 │   │   self.canvas.setCursor(type.to_qt())                            │
╰──────────────────────────────────────────────────────────────────────────────╯
Exception: Qt mouse button MouseButton.MiddleButton is unknown

just accidentally hit my middle mouse button and got an exception. In this case, it's easy enough to just map to MouseButton.MIDDLE, but more generally we should just fail silently if a mouse button we don't particularly care about is pressed (it shouldn't be an exception)