Use a new approach for multilang tags highlighting

The previous approach had some assumptions (e.g., it assumed there was
a singe instance of the editor on the same page) that lead to some
bugs and corner cases. Now the plugin hooks on the form submit event
and clean the highlighting <span> tags there. The cleaning doesn't use
any regexes any longer. Instead it uses DOM operations that are more
robust and potentially faster.

Also multilang tag highlighting <span>'s are now removed/added when
switching to/from HTML view, reducing clutter when editing the HTML code.

Some initial coding style cleanup work has been carried out. More to follow.

Tested in Firefox 56 and Chromium 61.0.3163.100.

Closes eljulians#2 eljulians#18 eljulians#21

default-css.php

@@ -22,12 +22,11 @@
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 
-$multilang2_default_css = <<< EOF
+defined('MOODLE_INTERNAL') || die();
 
+$multilang2defaultcss = "
 outline: 1px dotted;
 padding: 0.1em;
 margin: 0em 0.1em;
 background-color: #ffffaa;
-
-EOF
-;
+";

settings.php

@@ -30,4 +30,4 @@
     get_string('highlight', 'atto_multilang2'), get_string('highlight_desc', 'atto_multilang2'), 1));
 $settings->add(new admin_setting_configtextarea('atto_multilang2/customcss',
     get_string('customcss', 'atto_multilang2'), get_string('customcss_desc', 'atto_multilang2'),
-    $multilang2_default_css, PARAM_RAW));
+    $multilang2defaultcss, PARAM_RAW));

version.php

@@ -24,8 +24,8 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$plugin->version      = 2017052400;         // The current plugin version (Date: YYYYMMDDXX).
-$plugin->release      = 'v3.3.1.7 (version v1.7 for Moodle 3.3) (2016121100)';
+$plugin->version      = 2017100200;         // The current plugin version (Date: YYYYMMDDXX).
+$plugin->release      = 'v3.3.1.8 (version v1.8 for Moodle 3.3) (2016121100)';
 $plugin->requires     = 2017051500;         // Required Moodle version.
 $plugin->component    = 'atto_multilang2'; // Full name of the plugin (used for diagnostics).
 $plugin->maturity     = MATURITY_STABLE;