Add footprints in single layer and add assets of multiple items at once

[floriandeboissieu]

At the moment, footprints are added as separate layers with one polygon in each. When checking tens or hundreds of footprints, it populates the GUi with layers...

This PR adds the search result as a single vector layer, just saving the ItemCollection result as the geojson feature collection (which it is). This way it accelerates much the loading of the footprints and makes it much more manageable to play with (toggle display, remove layer, ...), while keeping the items info available.

A similar issue comes with the assets: at the moment, if I want to add the same asset, e.g. B02, from multiple items (e.g. tens) I have to enter the "View assets" dialog of each item to tick the "add layer" box and click on "Add assets".

This PR adds two buttons "Add selected items" and "Add all items" in order to be able to add the same assets for the items at once (without or after download, same as the View assets dialog)

A The same can be said for the assets, e.g. 10 tiles in the map canvas, that could be added

[floriandeboissieu]

closing and reopening in #271 with a more appropriate source branch