[FIX] web_editor: correctly display image and focus point in bg option dialog

Before this commit, the focus point the user can move on the image in
the background position configuration dialog was only following the
mouse as we want if the image original width was larger than the dialog
one. This is because the javascript relies on the fact the image takes
100% width of its container. To fix this, the container now does not
take 100% of modal width but the size of its internal image (with a
max-width of 100%).

addons/web_editor/static/src/less/web_editor.ui.components.less

@@ -243,7 +243,8 @@ body .modal {
                 font-weight: bold;
             }
             .o_bg_img_opt_object {
-                width: 100%;
+                display: inline-block;
+                max-width: 100%;
                 background: white;
                 min-height: 10px;
                 position: relative;