[GUI] Image API

[befl3]

Currently, the GUI is mostly tied to a specific source of background imagery (i.e., road geometry). Having the option to

• deactivate the background image (blank or single color background)

and/or to

• have the option to load arbitrary images as background

would be highly helpful. In the second case, an anchor coordinate (latitude/longitude) along with the size (height and width) of the depicted area in meters would need to be provided alongside the image.