Fix sub-category view still being visible when moving to a top-level kcm.
This fixes an issue where the sub-category view would still be visible when opening a different kcm that is at the top level. See bug #434345 for more details. For example, if you did: systemsettings kcm_mouse and then, while System Settings is still open on that page do: systemsettings kcm_printer_manager the sub category view on the left would still be on the Mouse & Touchpad page. This MR fixes that by resetting the pageStack index to 0.