ToC tweaks: always on ToC

- ToC button is always visible and enabled
- still: ToC opened automatically for all pages except Main page
- if user invokes ToC on Main page the onboarding text could show up
- show progress bar inside ToC drawer if ToC is opened while page is still loading

Change-Id: If9371dce97589d1c904fe1ca6c37057e74a433c0