erp5_xhtml_style: CKEditor – Fix duplicate image on paste

When pasting an image in CKEditor, the image was displayed twice.
This behaviour is due to the new paste-as-base64 feature implemented in CKE,
which conflict with existing plugins implementing the same feature.

To solve it, add `clipboard_handleImages` option to the CKE configuration object,
which disables its own image-pasting feature.

See https://github.com/ckeditor/ckeditor4/issues/4874 for the corresponding issue on CKEditor’s repository.

/cc @jerome @romain @tomo
/reviewed-by @jerome
/reviewed-on nexedi/erp5!1951

product/ERP5/bootstrap/erp5_xhtml_style/SkinTemplateItem/portal_skins/erp5_ckeditor/ckeditor.gadget.js.js

@@ -37,6 +37,8 @@
     MOBILE_CONFIGURATION = {
       toolbar: TOOLBAR_MOBILE,
       disableNativeSpellChecker: false,
+      // Image pasting is already handled by a plugin
+      clipboard_handleImages: false,
       // Remove the WebSpellChecker and SpellCheckAsYouType (SCAYT) plugins.
       removePlugins: 'scayt,wsc',
       // Disable ACF to not destroy HTML on mobile
@@ -48,6 +50,8 @@
     DESKTOP_CONFIGURATION = {
       toolbar: TOOLBAR_DESKTOP,
       disableNativeSpellChecker: false,
+      // Image pasting is already handled by a plugin
+      clipboard_handleImages: false,
       // Remove the WebSpellChecker and SpellCheckAsYouType (SCAYT) plugins.
       removePlugins: 'scayt,wsc',
       // Disable ACF to not destroy HTML on mobile
@@ -59,6 +63,7 @@
     READONLY_CONFIGURATION = {
       allowedContent: true,
       readOnly: true,
+      clipboard_handleImages: false,
       // Remove the WebSpellChecker and SpellCheckAsYouType (SCAYT) plugins.
       removePlugins: 'elementspath,scayt,wsc,toolbar,pastefromword,' +
                      'tableselection,uploadwidget,clipboard,pastetext,' +