Be more conservative when adding states to History API / make back button useful again

[lentinj]

Related to #940 & #547

Stop filling History API with every event we find by considering the current state either "fixed" (user wanted to visit this one) or "transient" (user is scrolling around).

When setting state, if the current state is "transient", then replace it with the next state we want. Otherwise add the state as a new state in the history API.

So, in more detail:

• Open treeviewer, the initial view is a "fixed" state
• Scroll around, we add a second "transient" state to the history list (i.e. back will go to initial view)
• Keep scrolling, we replace this "transient" state with your current location (i.e. back will still go to initial view)
• Clicking a node / leaf / search result will replace the current "transient" state with a "fixed" state pointing at the OTT1
• Clicking another node will add a second "fixed" state pointing at OTT2 (and back will go OTT1 -> initial)
• Clicking on signposts is a "transient" view, so multiple signpost-clicks don't fill the back button with noise.

@jrosindell @hyanwong does this sound like what we were talking about to you?

[lentinj]

Other things to think about:

• Where do settings changes figure in this, e.g. colour scheme changes? Currently they replace the existing state.
  • I think these are "transient" (i.e. as they are).
• What about tours? Currently they replace the existing state.
  • I think these are "fixed" (i.e. opposite to what currently happens, back in a tour stops the tour & takes you back to your last searched-for species, e.g.)
• What about popups? Currently they add 2 states, the existing state & a state with popup.
  • The mechanism for whether the pop-up is visible needs re-thinking in general, to stop the spurious-pop-up-on-reload Previously shown wikipedia page gets displayed again after searching for unrelated taxon #892.
  • EDIT: Do we even need to try and have pop-up state in the URL? Is being able to offer a link to treeviewer with a pop-up already open useful? I'm tempted to say no.
  • Otherwise, I think these are a "transient" state, i.e. clicking back will close the pop-up, and continuing to click back won't open closed pop-ups