ev-sidebar: preparations for GTK4 move and cleanup

ev-sidebar: Use stack children to keep record of the sidebar page includes a cleanup from !373 that can be mostly backported.

ev-sidebar: remove "active-icon-name" property is an unrelated cleanup. The property was introduced in 43c402ef to be used by ev-toolbar but later in other refactoring removed, since the icon for the toolbar was later considered fixed. Remove it since it is unnecessary. I have unfortunately not found the exact commit in which it is removed, but for sure not in use.

Relates #1864

Relates !373