Allow to provide custom logo icons to render in the title bar

[hholzgra]

Right now there's only support for an OpenStreetMap icon (always present), and a "Refugees Welcome" icon used by the neighborhood map application.

It should be possible to add a custom SVG icon to be rendered in addition, or as a replacement for, the OSM icon.

In addition to a local file path a HTTP download URL should also be supported

[hholzgra]

• [x] refactor code / remove duplicate code
• [x] support bundled:filename.svg syntax for bundled logo images
• [x] fetch logo and extra_logo from settings (URLs no longer hard coded)
• [ ] remove special handling for neighborhood POI maps
• [ ] support bitmap formats like PNG and JPEG, too, not just SVG
• [ ] support remote http:/https: URLs in addition to local files
• [x] add command line options --logo and --extra-logo