[Spatial selection] Allow to stop selection action "above" map anchor

[antoniave]

When using the selection, the user can draw an rectangle to perform a spatial selection. The user clicks in the map to start drawing the rectangle and when they release the mouse key, the drawing is stopped. However, when the mouse cursor is above an element placed in an map anchor, the drawing is not stopped. The user needs to click a second time somewhere on the map to stop the drawing.

It would be desirable that the user can also stop the selection drawing "above" an map anchor element.

Idea for implementation: Instead of placing map anchors "in" the map, let them render on top of the map.

[antoniave]

PR: #333