[osx] Fix window's setVisible(true)

[martincapello]

Fix aseprite/aseprite#4791

[martincapello]

I'm not actually sure if we should call setIsVisible always...if this were the case, then we can just do:

[m_nsWindow setIsVisible:visible]

At the end of WindowOSX::setVisibleand remove the elses.

Just tried and the following seems to work as well:

void WindowOSX::setVisible(bool visible)
{
  if (visible) {
    // The main window can be changed only when the NSWindow
    // is visible (i.e. when NSWindow::canBecomeMainWindow
    // returns YES).
    if (m_nsWindow.canBecomeMainWindow)
      [m_nsWindow makeMainWindow];
  }
  [m_nsWindow setIsVisible:visible];
}

[dacap]

It works great! 👍