Add noVNC_setting_warn_before_close

Ability to to toggle the warning prompt before closing the tab or window introduced by 63c2c14a50 (PR #2006).

Closes #2014

app/ui.js

@@ -184,6 +184,7 @@ const UI = {
         UI.initSetting('shared', true);
         UI.initSetting('bell', 'on');
         UI.initSetting('view_only', false);
+        UI.initSetting('warn_before_close', false);
         UI.initSetting('show_dot', false);
         UI.initSetting('path', 'websockify');
         UI.initSetting('repeaterID', '');
@@ -369,6 +370,7 @@ const UI = {
         UI.addSettingChangeHandler('shared');
         UI.addSettingChangeHandler('view_only');
         UI.addSettingChangeHandler('view_only', UI.updateViewOnly);
+        UI.addSettingChangeHandler('warn_before_close');
         UI.addSettingChangeHandler('show_dot');
         UI.addSettingChangeHandler('show_dot', UI.updateShowDotCursor);
         UI.addSettingChangeHandler('host');
@@ -887,6 +889,7 @@ const UI = {
         UI.updateSetting('compression');
         UI.updateSetting('shared');
         UI.updateSetting('view_only');
+        UI.updateSetting('warn_before_close');
         UI.updateSetting('path');
         UI.updateSetting('repeaterID');
         UI.updateSetting('logging');
@@ -1221,8 +1224,10 @@ const UI = {
         // preventDefault() or a nonempty returnValue, though the latter is
         // considered legacy. The custom string is ignored by modern browsers,
         // which display a native message, but older browsers will show it.
-        e.preventDefault();
-        e.returnValue = _("Are you sure you want to disconnect the session?");
+        if (UI.getSetting('warn_before_close')) {
+            e.preventDefault();
+            e.returnValue = _("Are you sure you want to disconnect the session?");
+        }
     },
 
     updateBeforeUnload() {
@@ -1769,6 +1774,7 @@ const UI = {
                 .classList.add('noVNC_hidden');
             document.getElementById('noVNC_clipboard_button')
                 .classList.add('noVNC_hidden');
+            UI.disableSetting('warn_before_close');
         } else {
             document.getElementById('noVNC_keyboard_button')
                 .classList.remove('noVNC_hidden');
@@ -1776,6 +1782,7 @@ const UI = {
                 .classList.remove('noVNC_hidden');
             document.getElementById('noVNC_clipboard_button')
                 .classList.remove('noVNC_hidden');
+            UI.enableSetting('warn_before_close');
         }
     },
 

vnc.html

@@ -219,6 +219,13 @@
                             View only
                         </label>
                     </li>
+                    <li>
+                        <label>
+                            <input id="noVNC_setting_warn_before_close" type="checkbox"
+                                   class="toggle">
+                            Warn before close
+                        </label>
+                    </li>
                     <li><hr></li>
                     <li>
                         <label>