iviewer multiviewers on double-click

[will-moore]

This continues the design from #79
focusing only on the opening of viewers with double-click.

As discussed with @pwalczysko (see his last comment #79 (comment)) we can simplify the desired behaviour (no 'pinning', 'temporary windows' etc):

• No single vv multi-viewer mode.
• When a single viewer is open, behaviour is same as current behaviour.
• Single-clicking on a thumbnail opens the image in the currently-active viewer (does nothing if current viewer is already showing that image).
• Double-click on a thumbnail will open the image in a NEW viewer panel.
• When closing viewers, when only a single one is open it reverts to filling the space (as current behaviour).

cc @waxenegger @jburel

[waxenegger]

@will-moore @jburel distinction between single and double clicks in terms of event handling is an undetermined nuisance. In essence a double click is very likely to cause a single click event first, after that a double click event is fired. According to some sources out there some browsers could even do it the other way round. But even if we assume it's the former which is the behavior I get one needs a timer solution to handle that. Hacky at best. Here is what the jquery doc has to say: https://api.jquery.com/dblclick/ (but it's really something that is problematic based on generic event handling, not jquery, I wouldn't even be using jquery in that case)

[jburel]

Another option could be to drag a thumbnail from left panel to centre panel
to open in multiview