Left click on tray icon always raises main window

**Problem**

It seems impossible to prevent a left-click on a tray icon from raising the main window. To reproduce this, modify the `multiwindow_with_tray_icon.rs` example to have the following:

```rust
let tray = init_tray_icon(default_tray_icon(), None);
tray.set_show_menu_on_left_click(true);
```

Run the example, close the main window, and left-click on the tray icon. Note that the context menu appears AND the main window re-appears. I've tried many different ways to suppress this behaviour, and nothing has worked.

Platform: macOS, reproduced with 0.7 alpha 3.

**Expected behavior**

With the code above, only the context menu for the tray appears. I also think that raising the main window by default on left-click is a misfeature that may cause confusion (if it's intentional).