feat(seo): add meta keywords for supported languages

- keyboard-details.php: use the langauge name(s) to produce keyboards.
- template.php: support 'keywords' to pass to head()
- Head.php: to support keywords as a parameter to head(), rendered as a meta tag. Escape ASCII quotes properly as entities.

_includes/2020/templates/Head.php

@@ -42,6 +42,10 @@ static function render($fields = []) {
   if(KeymanHosts::Instance()->Tier() == KeymanHosts::TIER_STAGING) {
     echo '    <meta name="robots" content="none">';
   }
+  if(isset($fields->keywords) && !empty($fields->keywords)) {
+    $keywords = htmlspecialchars($fields->keywords, ENT_QUOTES, "UTF-8");
+    echo "<meta name=\"keywords\" content=\"" . $keywords . "\" />\n";
+  }
 ?>
   <title><?= $fields->title; ?></title>
   <?php
@@ -81,4 +85,4 @@ static function render($fields = []) {
 
 <?php
     }
-  }
+  }

_includes/includes/template.php

@@ -15,7 +15,7 @@ function template_finish($foot) {
   }
 
   function head($args=[]){
-    // Args are title='My Page Title', css='page.css' showMenu=true/false, showHeader=true/false, foot=true/false
+    // Args are title='My Page Title', css='page.css' showMenu=true/false, showHeader=true/false, foot=true/false, keywords='comma,separated,list'
 
     $agent = isset($_SERVER['HTTP_USER_AGENT']) ? $_SERVER['HTTP_USER_AGENT'] : '';
     // Get device
@@ -55,6 +55,9 @@ function head($args=[]){
     if(isset($args['language'])) {
       $language = $args['language'];
     }
+    if(isset($args['keywords'])) {
+      $keywords = $args['keywords'];
+    }
     if(isset($args['css'])){
       $css = array();
       foreach($args['css'] as $cssFile){
@@ -91,6 +94,7 @@ function head($args=[]){
     if(isset($language)) $head['language'] = $language;
     if(isset($css)) $head['css'] = $css;
     if(isset($js)) $head['js'] = $js;
+    if(isset($keywords)) $head['keywords'] = $keywords;
     \Keyman\Site\com\keyman\templates\Head::render($head);
 
     if($menu == true) {

_includes/includes/ui/keyboard-details.php

@@ -268,6 +268,22 @@ protected static function WriteTitle() {
         ];
         $embed_target = '';
       }
+
+      // calculate keywords per language
+      $keywords = '';
+      if (isset(self::$keyboard->languages)) {
+        foreach (self::$keyboard->languages as $lang) {
+          if (!empty($keywords)) {
+            $keywords = $keywords . ', ';
+          }
+          $languageName = $lang->languageName;
+          $keywords = $keywords . $languageName . ' Keyboard';
+        }
+      }
+
+      // will be ignored if ''
+      $head_options['keywords'] = $keywords;
+
       head($head_options);
 
       if($embed == 'none') { ?>