Option to disable background entirely

[tmcw]

So opacity: 0% still loads tiles. We should have a 'None' option for the background and/or have setting opacity to 0 have the same effect.

[pnorman]

As a tile server admin I'm in favour of the latter (although maybe the former should be done too). It does break a workflow with hiding the background while loading from an extremely slow TMS server, but that's a very obscure workflow and if you're doing that you're probably best using JOSM with it's more aggressive TMS caching anyways.

[tmcw]

Yep, definitely a nice-to-have. Not seeing this as a launch blocker in any way.