Shut down Konqueror even when "always have preloaded instance" option is on
When the "always have preloaded instance" option is turned on, it's impossible to close Konqueror without recurring to kill or something like it. This happens because there'll always be an invisible instance of KonqMainWindow which the user won't be able to close.
While having a window ready is what preloading is about, it causes an unexpected issue. For example, if the user launches Konqueror from a terminal, control won't return to the shell after closing the last window.
This change makes Konqueror close the preloaded window when the last visible window is closed, which causes the application to close. After that, if the "always have a preloaded instance" option is on, a new instance of Konqueror is launched using the --preload argument.