First naive version of datalayer grouping

[yohanboniface]

A few questions/remarks:

• we are using datalist so groups name have to be written only once and will then appear in autocomplete, but if one wants to reword a group name, they will need to reword it once, but then go to all datalayers of this group to select the new value
• the "toggle all" button does not reflect the current state of the visibility of the group (tricky: all visible, all hidden, or mixed)
• at this stage, datalayers without a group are not displayed any more in the browser, letting users choose either to have a group "Others" or let them hidden, but maybe we want something more automatic
• we do not reflect groups yet in the caption panel, but we should do if this is the direction we want to take
• going this way, we touch the idea of also changing colors and settings for a whole group at once, so that would mean a sort of datalayer hierarchy, but this would open a lot of questions (perms being maybe the tricky one)
• no integration tests yet as I'm not sure yet it's the way to go