Add full layer control to composer

[anitagraser]

It would be convenient to have direct control over each map item's layers and be able to remove / add layers from a map item.

As discussed on https://twitter.com/madmanwoo/status/804695847384399872

[ponceta]

This is almost possible if you work on two screens or split screen but you will modify your qgis project styles and rendering order which is not what you want.

What could be great would be to have these tools available for each map canvas in the composer :)

And settings would apply to each map canvas separately.