Save user preferences for geolocation

See prompting for permission: https://developer.mozilla.org/en-US/docs/Web/API/Geolocation_API

```function prompt(window, pref, message, callback) {
let branch = Components.classes["@mozilla.org/preferences-service;1"]
.getService(Components.interfaces.nsIPrefBranch);

if (branch.getPrefType(pref) === branch.PREF_STRING) {
switch (branch.getCharPref(pref)) {
case "always":
return callback(true);
case "never":
return callback(false);
}
}

let done = false;

function remember(value, result) {
return function() {
done = true;
branch.setCharPref(pref, value);
callback(result);
}
}

let self = window.PopupNotifications.show(
window.gBrowser.selectedBrowser,
"geolocation",
message,
"geo-notification-icon",
{
label: "Share Location",
accessKey: "S",
callback: function(notification) {
done = true;
callback(true);
}
}, [
{
label: "Always Share",
accessKey: "A",
callback: remember("always", true)
},
{
label: "Never Share",
accessKey: "N",
callback: remember("never", false)
}
], {
eventCallback: function(event) {
if (event === "dismissed") {
if (!done) callback(false);
done = true;
window.PopupNotifications.remove(self);
}
},
persistWhileVisible: true
});
}

prompt(window,
"extensions.foo-addon.allowGeolocation",
"Foo Add-on wants to know your location.",
function callback(allowed) { alert(allowed); });```