Allow user to store a set of features for later geofencing

[ahmadayubi]

This requires collaboration between https://github.com/Maps4HTML/Web-Map-Custom-Element and the extension

The idea brought up by @prushforth was as follows:

• allow the user to store a set of features from a MapML viewer for offline use
• allow the user to set a threshold, once the user is within that threshold of a feature from the stored sets, notify the user they are near the given feature
• allow the user to delete sets
• allow the user to turn off notifications for the various sets separately