Improve base-layers panel

[gioman]

Base layers panel could be improved so to allow add thumbnails for each map, to help users better understand what is the content of each base layer. Also the panel could be visible only when necessary (by adding an icon in the toolbar), and once a base layer is set the panel could disappear leaving more space for more prominent things, as the TOC, example:

[Gustry]

This is already possible with an external javascript, but for now, indeed it's not in core.