Add QKeySequence to VirtualDesktopManager::addAction overload

There are four usages of this overload, two of them are for the
intentionally disabled "Switch to Next/Previous Desktop" actions and the
rest are for "Switch One Desktop to X" actions. Due to the order they
were added, an empty keybind was set as the default and the actual
keybind is never enabled.

Now there's a QKeySequence argument to this overload, so an unexpected
empty keybind is never added. The two usages of addAction that depend on
this empty keybind behavior now pass in an empty QKeySequence.

BUG: 475748


(cherry picked from commit e3982892)