Revert 4fd1a2b2d4.

Fixes double free crash at exit.

We can't use QWidgets as K_GLOBAL_STATICs. QWidget's dtor
dereferences other global pointers (X11 stuff), which might,
or might not already been destroyed.

Works well when K_GLOBAL_STATIC deletes the widget before those
global pointers are destroyed, but we can't control the order.

Till, what about doing the caching inside the dialog's implementation?