Cleanup old explorer resources and API

There's a number of resources and API that are not used in the explorer:

• there was a "tree" display, some server-side computation is also still performed --> remove all this
• some templates reference "hideNav" to remove header tabs, but these are not shown anyway when using embedded mode --> clean them up

More generally the main tabs should be included in a specific template for subviews of a given distribution, but i gave up on changing this odd strategy.
Renaming block in the main macro to give them a semantic name instead of a position name ("middle", "right") would also be less confusing since these elements have moved in the page, over time, but i kept them like this anyway...