Modal language selector on the welcome page

The language selector on the welcome page has been replaced with
a smaller button that opens a modal language selector. Though the
code for introducing such a modal language selector has been added
in i18n.js, its appearance relies on styles defined in index.css.

Once this new UI for changing the UI language is approved, it must be
used in the ZIM viewer too.

Known issues:

- selecting the language with arrow keys (using the keyboard only,
  without pressing space first, so that the full list of languages is
  shown) doesn't work because as soon as the current language is changed
  the modal language selector disappears.

static/templates/index.html

@@ -52,9 +52,18 @@
            aria-label="Library OPDS Feed"
            title="Library OPDS Feed">
     </a>
+    <a onclick="window.modalUILanguageSelector.show()">
+      <svg xmlns="http://www.w3.org/2000/svg"
+           id="uiLanguageSelectorButton"
+           width="30"
+           height="30"
+           viewBox="0 0 20 20">
+        <g fill="#36c">
+          <path d="M20 18h-1.44a.61.61 0 0 1-.4-.12.81.81 0 0 1-.23-.31L17 15h-5l-1 2.54a.77.77 0 0 1-.22.3.59.59 0 0 1-.4.14H9l4.55-11.47h1.89zm-3.53-4.31L14.89 9.5a11.62 11.62 0 0 1-.39-1.24q-.09.37-.19.69l-.19.56-1.58 4.19zm-6.3-1.58a13.43 13.43 0 0 1-2.91-1.41 11.46 11.46 0 0 0 2.81-5.37H12V4H7.31a4 4 0 0 0-.2-.56C6.87 2.79 6.6 2 6.6 2l-1.47.5s.4.89.6 1.5H0v1.33h2.15A11.23 11.23 0 0 0 5 10.7a17.19 17.19 0 0 1-5 2.1q.56.82.87 1.38a23.28 23.28 0 0 0 5.22-2.51 15.64 15.64 0 0 0 3.56 1.77zM3.63 5.33h4.91a8.11 8.11 0 0 1-2.45 4.45 9.11 9.11 0 0 1-2.46-4.45z"/>
+        </g>
+      </svg>
+    </a>
     <div class='kiwixNav'>
-      <select id="ui_language">
-      </select>
       <div class="kiwixNav__filters">
         <div class="kiwixNav__select">
           <select name="lang" id="languageFilter" class='kiwixNav__kiwixFilter filter'>
@@ -85,7 +94,11 @@
   <script>
     function closeModal() {
       for(modal of document.getElementsByClassName('modal-wrapper')) {
-        modal.remove();
+        if ( modal.id == "uiLanguageSelector" ) {
+          window.modalUILanguageSelector.close();
+        } else {
+          modal.remove();
+        }
       }
     }
   </script>