New: Credits dialog

Added copyright information / credits dialog into the main menu.
Inspired by [tabascoeye/workadventure](7b7c695f76), requested by Chaoschemnitz

Credits dialog shows a text field. It's content should be provided by map authors
by defining a map property of type 'string' with the unique key 'license'.

Cleanup of the main menu:
- removed outline from button:active
- button paddings/margins
- removed "Create map" and "Share url"
- renamed buttons "Edit name" / "Edit skin" into "Name" / "Avatar" for
better DE support

front/dist/resources/html/gameMenu.html

@@ -6,11 +6,17 @@
     * a, button, select{
         cursor: url('/resources/logos/cursor_pointer.png'), pointer;
     }
+    #gameMenu {
+        padding-top: 8px;
+    }
     #gameMenu button {
         background-color: black;
         color: white;
         border-radius: 7px;
-        padding-bottom: 2px;
+        padding: 2px 8px;
+    }
+    #gameMenu button:active {
+	outline: none;
     }
     #gameMenu section {
         margin: 10px;
@@ -20,21 +26,24 @@
 <div id="gameMenu" hidden>
 
     <main>
-        <section>
+        <section hidden>
             <button id="shareButton">Share url</button>
         </section>
         <section>
-            <button id="changeNameButton">Edit name</button>
+            <button id="changeNameButton">Name</button>
         </section>
         <section>
-            <button id="changeSkinButton">Edit skin</button>
+            <button id="changeSkinButton">Avatar</button>
         </section>
         <section>
             <button id="editGameSettingsButton">Settings</button>
         </section>
-        <section>
+        <section hidden>
             <button id="sparkButton">Create map</button>
         </section>
+        <section id="copyrightSection">
+            <button id="copyrightButton">Credits</button>
+        </section>
         <section id="adminConsoleSection" hidden>
             <button id="adminConsoleButton">Admin console</button>
         </section>