Port away from KIconThemes

Use QIcon instead of QPixmap where possible. This avoids having to specify a size in some places.

There is a behavior change. The old code clearly intends to fall back to specific icons when the requested name is not available. However this does not work since the first call to QIcon::fromTheme will always give a non-null pixmap since it has a builtin fallback. The new code correctly makes use of the fallback icons.