Improve sidebar behavior, and fix loading a specified kcm to start
Improve the sidebar switching behavior such that the kcm is kept in memory rather than being recreated when the window changes mode.
Also fix behavior when opening a non-existent kcm at start (just show regular window rather than blank page).
Also fix behavior when opening a kcm at start in mobile mode, so that the kcm isn't popped instantly from the page stack.