feat(core,fuse,gui): places on a world map

The Places view gets a Grid/Map toggle. The map shows a bubble per town,
sized by its number of photos, over the outline of the world's land. Towns
whose bubbles would overlap merge into one that counts all their photos, and
clicking it zooms in until they part; clicking a single town opens it like a
card does. Drag to pan, scroll or pinch to zoom, and the buttons zoom in,
zoom out or frame every place again. Hovering names the town, or the first
few towns of a merged bubble.

No tile server is involved: none that is free has terms allowing an app like
this one to use it. The map is drawn from Natural Earth's 1:50m land
polygons (public domain), trimmed and simplified by
scripts/natural-earth-land.py into crates/pdfs-gui/data/land.txt, in Web
Mercator. The About dialog credits Natural Earth.

PlaceInfo carries the town's latitude and longitude; a daemon older than the
map sends none, and its towns are left off the map.

crates/pdfs-core/src/control.rs

@@ -1456,6 +1456,12 @@ pub struct PlaceInfo {
     pub name: String,
     /// ISO 3166-1 alpha-2 code of the town's country.
     pub country: String,
+    /// Where the town is, in decimal degrees, for the map. Zero from a daemon
+    /// older than the map.
+    #[serde(default)]
+    pub latitude: f64,
+    #[serde(default)]
+    pub longitude: f64,
     /// How many photos were taken there, counting a RAW+JPEG shot once.
     pub photo_count: usize,
     /// The newest photo taken there, shown as the place's cover.