[GUI] alert user if loading/save is done

[szlldm]

If the application is put to the background, it alerts the user that long lasting loading/saving is done
According to Qt documentation of QApplication::alert :

On macOS, this works more at the application level and will cause the application icon to bounce in the dock.

On Windows, this causes the window's taskbar entry to flash for a time. If msec is zero, the flashing will stop and the taskbar entry will turn a different color (currently orange).

On X11, this will cause the window to be marked as "demands attention", the window must not be hidden (i.e. not have hide() called on it, but be visible in some sort of way) in order for this to work.