Fix x264 stdint.h message (it's already included)
Make application quit on main window close

This causes the main window to signal the application to exit and delete
its own pointer on close.  This fixes an issue where apparently some
windows that aren't explicitely connected to the main window would be
left open when the main window was closed because by default Qt will
only exit when all windows have been closed.

Because it deletes its own pointer, instead of storing it in a
std::unique_ptr, use a QPointer because it has built-in support for
automatically has an internal mechanism for tracking QObject deletion
even if the deletion was not done on the QPointer itself, where as
unique_ptr does not have that functionality.  In other words, if the
pointer is deleted elsewhere for whatever reason, the QPointer will
still set that internal value to null.
Minor refactor for creation of sub-windows

This refactors the sub-window code a bit so that instead of deleting the
window pointers, it calls QWidget::close() on them to safely trigger a
normal close on them instead (which will also delete them).  It adds an
OBSShutdown signal which will emit when the main window is being closed.