Cleanup: Update headerbar

[jeremypw]

Start to simplify keeping the headerbar in sync without calling update_headerbar many times unnecessarily.

• Merge another related function into update headerbar
• Only necessary to connect to the active signal for Miller views as only these can change location without loading
• Fixed keeping track of restoring_tabs and added another condition for ignoring update_headerbar
• Stop triggering a load (and update) when a window is associated with a view container - it is unnecessary and overloads the function
• Add some explanatory comments

In passing restoration of the active tab position was fixed. Fixes #2811

Hopefully the rather complicated interface between Window and ViewContainer can be be further simplified in later PRs.