Add a SelectionContext

[mmaelicke]

stuff to finish the selection context:

• add a basic skeleton context for selections #44
• add offline context handlers to save and restore selections #45
• map selection state from offline to selection context #56
• add import and export functions for selections
• add function to add the current filter to the active selection
• add function to create a new selection from the current filter
• add function to add trees manually to the current selection #65
• add function to remove a tree from the current selection #80
• add experimental component to view selections (ie. for debugging the developments) #55
• add a selection-list component #84
• add a current selection component
• add a popover and a modal to view the selection-list
• add a layer to the map for selections #66