Cache the icons used in the completion list, and force them to be loaded by converting them to a pixmap, and then back to an icon.

This was necessary because on my quite ok computer(Athlon 64 3200+), the completion list has become unusably slow due to permanent icon loading.

Since we're supposed to have an icon cache, and it was faster before, this seems like a bug in KDE.
However, it will always be faster creating and loading the icons statically like this.

For me this makes the completion-list usable again.