Add new bookmark function, to also store viewport

[IanMayo]

From the time-controller drop-down menu we can add a bookmark for the current DTG.

It would be useful to also add the ability to bookmark both the DTG and the viewport.

So - add a new type of bookmark which also stores the viewport bounds.

When the user clicks on it - move to both the specified DTG and viewport.

Stretch goal

With a collection of bookmarks, an analyst could move between them in sequence. It would be attractive if we could animate the transition. So, if a bookmark is in the same plot as the one that's currently open, we would produce time-delta and location-delta values. We'd then slice each into (say) 10 steps - and then apply those time/location step values. This would effectively give a dynamic transition between the points.

Stretching that a bit further... If we allow a 2-second transition time, we could time the length of the first step, then recalculate the number of steps in the remaining delta, so it always takes around 2 seconds.

Hold on, the above may not be possible. We need a "step-size" to perform the first step, before we have chance to re-calculate how long each step should be. Hmm, maybe start off with a 10% step, then calculate the step-size for the remaining steps based on the time taken.

[ayeshare]

#5153