Add Overlay
Created by: slifty
This PR adds a new overlay feature to SimpleMaps, allowing markers to have an overlay that is rendered when a user hovers their mouse on a marker feature.
This adds two new local settings:
Overlay TitleOverlay Default
Which determine map-wide content for the overlay.
It also adds a new marker setting Overlay Content which defines what content to render when hovering over a marker's feature.
Resolves #10 (closed)