Provide a selection manager component

[claustres]

At the present time the selected items are not listed in a component from which actions could be taken. They are highlighted on the map and some actions are possible through a widget on the last selected item (eg information box) but this is not sufficient to perform individual or group actions like geometry or style editing.

Related to #1097 as edition restricted to a specific set of features could be triggered from this new component. As editing is only possible on a specific layer right now it would be interesting to group features by layer in the selection manager using a tree (layer is the parent node of selected features from the layer). The edit action should then be available on the layer element or a single feature element but not between features from different layer elements.

Related to #1072 as the style of a feature could be edited from this new component as well.

Related to #1106 for highlights.

• features selection display
• features properties display
• zoom to action
• edit geometry action
• edit properties action
• reset style action
• edit style action
• remove action
• manage in-memory layers

[claustres]

Should be close once style-manager branch merged.