Fixed: Trying to open a CPPopover in a secondary platform window opened in the main window

Previously, when opening a popover on a view from another platform window, the popover
was actually opened in the main window. This fix ensure to set the correct platform window
to the _CPPopoverWindow by setting it to the [[positioningView window] platformWindow];
Fixed: CPAlert always shows on main platform window

Previously, a CPAlert was always shown on the main platform window. This behavior could result in a feeling that the application was hanging because of the modal nature of the alert when users are looking to another platform window. This fix ensure to set the CPAlert window's platformWindow to the current key window using [[CPApp keyWindow] platformWindow].