Add Settings panel with a button to change themes

Only 2 themes besides the default are implemented for now.
More to come!

sources/scripts/settings.js

@@ -0,0 +1,22 @@
+'use strict'
+
+function Settings () {
+  this.element = null
+  this.theme_button = null
+
+  this.install = function () {
+    this.element = document.createElement('div')
+    this.element.setAttribute('class', 'settings')
+
+    this.theme_button = document.createElement('select')
+    this.theme_button.setAttribute('class', 'theme button')
+    this.theme_button.innerHTML = '<option value=\"theme\">Theme (default)</option>'
+    this.theme_button.innerHTML += '<option value=\"gameboy\">Game Boy</option>'
+    this.theme_button.innerHTML += '<option value=\"nightowl\">Night Owl</option>'
+    this.theme_button.addEventListener('mousedown', () => { donsol.player.change_theme(this.theme_button.value) })
+
+    this.element.appendChild(this.theme_button)
+
+    return this.element
+  }
+}