Add preferences page

* For now, the prefs page has just a single option, Downloads Folder
* For now, you can't type in a folder, you must use the chooser
* Further fixes coming om master
* Written by @ChrisMorrisOrg and @grunjol, rebased by @dcposch

renderer/views/header.js

@@ -37,7 +37,7 @@ function Header (state) {
   }
 
   function getAddButton () {
-    if (state.location.url() !== 'player') {
+    if (state.location.url() === 'home') {
       return hx`
         <i
           class='icon add'