Partly revert "Resolves: #ii122335# Disabling the old task pane"

This partly reverts commit 95ae39c0c34c9a5e6fa6e72bf3a631a9a799b872.

SideBar is experimental, Impress still needs the TaskPane slot, otherwise once
disabled / clicked away the user won't get the TaskPane back.

Change-Id: I65b9e4a52ef2f3f1e7240fc01b828d2ff5d1589c
Reviewed-on: https://gerrit.libreoffice.org/6943
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>