Add "Get more Apps in Discover" Button to kopenwithdialog

Discover has a neat Feature: If you call it with the --mime Argument, it shows all Apps that can open this Mimetype. This MR adds a Button t the Open With Dialog, which opens Discover with a List of all Apps that can open the selected Mimetype. The Button should not be shown, when Discover is not installed, but I don't know if there is already a function for checking this or if I need to write my own.