Render shared events

[GriffinRichards]

Some maps share events with other maps, and have a shared_events_map field in map.json to indicate the name of the map that owns the events. At the moment opening a map with shared events shows no events on the map. Adding new events to these maps appears to work, but the changes are ultimately discarded and will disappear when the project is reloaded.

It might be nice if instead a map that shares events renders those events and allows them to be edited. At a minimum I'd consider it a bug that it will discard events, and that there's no indicator that the map shares events.

(Similarly maps can share scripts files. Any map that shares scripts but doesn't own the file will have no scripts populated in the Script dropdowns, and Open Map Scripts will do nothing. These should be redirected to the shared file).