Refactoring preloading: no need for a kded module anymore.

The DBus service name org.kde.konqueror.preloaded indicates
which process can be used to quickly open a new window.
The first konqueror instance ensures that a --preload process runs
and as soon as the preloaded window is used, another --preload process
is started to handle the next request. To make even the very first
one fast, one can set the "start preload instance on startup" checkbox.

GUI: performance config dialog: the max instances count spinbox is gone,
and "always ensure one preload instance exists" is on by default, to make
the above actually work out of the box.