Add display of drawn features

[silvester-pari]

After the user draws some features in the eox-drawtools, they should be rendered as a list.
The user should also be able to select the features from the list (might need some additions in eox-map) or delete individual features. On feature select it would be nice if the map would zoom to that feature.
When all features are discarded, also this list should be cleared.

[silvester-pari]

Implemented in #371