Migrate all the non react components code to typescript (#847)

This completes the migration to typescript of all the non react
components code.
The only changes introduced besides types are the type checks using
`"something" in object` which narrows down types in typescript.

src/libs/metadata.ts

@@ -0,0 +1,41 @@
+import npmurl from 'url'
+
+function loadJSON(url: string, defaultValue: any, cb: (...args: any[]) => void) {
+  fetch(url, {
+    mode: 'cors',
+    credentials: "same-origin"
+  })
+  .then(function(response) {
+    return response.json();
+  })
+  .then(function(body) {
+    cb(body)
+  })
+  .catch(function() {
+    console.warn('Can not metadata for ' + url)
+    cb(defaultValue)
+  })
+}
+
+export function downloadGlyphsMetadata(urlTemplate: string, cb: (...args: any[]) => void) {
+  if(!urlTemplate) return cb([])
+
+  // Special handling because Tileserver GL serves the fontstacks metadata differently
+  // https://github.com/klokantech/tileserver-gl/pull/104#issuecomment-274444087
+  let urlObj = npmurl.parse(urlTemplate);
+  const normPathPart = '/%7Bfontstack%7D/%7Brange%7D.pbf';
+  if(urlObj.pathname === normPathPart) {
+    urlObj.pathname = '/fontstacks.json';
+  } else {
+    urlObj.pathname = urlObj.pathname!.replace(normPathPart, '.json');
+  }
+  let url = npmurl.format(urlObj);
+
+  loadJSON(url, [], cb)
+}
+
+export function downloadSpriteMetadata(baseUrl: string, cb: (...args: any[]) => void) {
+  if(!baseUrl) return cb([])
+  const url = baseUrl + '.json'
+  loadJSON(url, {}, glyphs => cb(Object.keys(glyphs)))
+}