TMS layers don't zoom beyond z18 by default

[rasher]

By default, TMS layers won't zoom past z18 unless the user manually adds a maximum zoom level in the "Zoom" column of the layer dialog. This seems slightly unfortunate, as this interface isn't very obvious.

I suspect #6134 might help for manually added layers (I have only read the description of the patch), but for layers added from a preset, the problem still occurs.

On the Settings tab, there is a max zoom level setting, which appears to have no effect at all - despite setting it to 20, a TMS layer with no max zoom set will still only zoom until z18.

It would be nice if:

• The presets from wiki:Maps could include a (default) max zoomlevel
• If no max zoom level is set for a layer, either the setting from the settings tab should be used, or perhaps the imagery plugin should be smart enough to allow zooming until it receives a 40x response.