Fixed mouse clicks not activating windows (Win32)

- This fixes issue #437, except Alt+F4, which would be a different issue?
- This is essentially based around @All8Up's suggestion.
- The new member variable `m_focused` is meant to be exposed later on so the user is able to determine whether the window is active without doing separate event tracking.
- Activating mouse clicks aren't passed to the event queue. This is something not really consistent across games or applications in general, but I prefer this behavior (avoids triggering anything by accident).