#246 only select image if its object is not in bin

A Stadtbild of a image can only be selected/deselected if its
Sb_stadtbildserieGridObject not in the bin.
 #246 refactored Sb_SingleStadtbildJGrid
 #246 check source of event

In Sb_stadtbildserieAggregationRendererInfoPanel check if the currently
shown Sb_stadtbildserieGridObject is the source of the fired methods
stadtbildChosen() and stadtbildUnchosen()
 #246 added a button to download all selected images

In Sb_stadtbildserieAggregationRenderer a button was added which allows
the user to download all selected Stadtbilder. This button is only
enabled, if at least one image is accessible. This means that a high-res
picture must exist and the image must not be intended for the internal
usage.
 #246 comments and refactoring