Change context of popup view

Make the context of the view the marker, and if you are in need of getting information on the map, then reference controller

Consider a marker has content
```
{
  location: latLng(12,34),
  title: "sample title"
}
```

In the old way, the view has to access these variables like: (zoom is coming from the leaflet map object)
```
<h1>{{view.content.title}}</h1>
{{view.content.lat}} {{view.content.lng}} at zoom {{zoom}}
```

This PR changes that into
```
<h1>{{title}}</h1>
{{lat}} {{lng}} at zoom {{controller.zoom}}
```

Which makes much more sense to me