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 Title
• Overlay 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)