Draft: ev-sidebar-*: remove "main-widget" property and iit's use in ev-window

This introduces way too many conflicts, so wait for GTK4

This was introduced in 4f1722b0 for bindings to work in the sidebar. This is no longer an issue, and bindings keep working in the sidebar regardless of this. Probably now with GTK3 the events are sent correctly to the focused widgets, although it's hard to tell why this workaround is unnecessary when it was introduced 18 years ago with little additional information around it (like a bugzilla ticket).