[BUGFIX] Use loading="lazy" in FormEngineUtility::getIconHtml()

The method `FormEngineUtility::getIconHtml()` is used in various places,
e.g. when rendering icons of a SelectIcons field wizard. If many icons
are loaded, the rendering process may take some time, depending on the
user's internet connection. To improve the initial rendering speed, the
icons are now lazy loaded.

Resolves: #94877
Releases: master, 10.4
Change-Id: I1ffb395124a7d9793188c7acdb039c3da698d41c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70573


Tested-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Frank Nägler <frank.naegler@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frank Nägler <frank.naegler@typo3.com>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>

typo3/sysext/backend/Classes/Form/Utility/FormEngineUtility.php

@@ -125,6 +125,7 @@ class FormEngineUtility
         $absoluteFilePath = GeneralUtility::getFileAbsFileName($icon);
         if (!empty($absoluteFilePath) && is_file($absoluteFilePath)) {
             return '<img'
+                . ' loading="lazy" '
                 . ' src="' . htmlspecialchars(PathUtility::getAbsoluteWebPath($absoluteFilePath)) . '"'
                 . ' alt="' . htmlspecialchars($alt) . '" '
                 . ($title ? 'title="' . htmlspecialchars($title) . '"' : '')