Map info: fix icons
The icon in the map info pane always shows an interrogation mark, even when the icon is actually defined (eg. show in the layer selection tree).
This involves changing the internal naming convention for icon classes, which is still inherited from an earlier version, where the store (database schema) wasn't used. This will also eliminate conflicts when the same layer name is used for different schemas.
Fixes #9519: use full store name for icon classes
Display icon in info pane
Cleanup: drop get_icon and get_symbol