[FIX] web_editor: restore selected images display

In the media dialog, when an image is selected, it has a colored border
to indicate it. Commit https://github.com/odoo/odoo/commit/e011d58bee140411ccfb1ef9115dddc090d573c3
made changes, part of them designed to "Improve the image dialog to be
able to select/unselect". While this commit was indeed doing that, it
was also breaking stuff: when selecting image(s) then performing a
search, the colored border disappeared while the images were in fact
still selected.

Fortunately, this stuff breaking is only due to code modification that
was made for no reason (so reverting part of the commit is enough to
restore the features while keeping the commit's improvements).

addons/web_editor/static/src/js/widgets/widgets.js

@@ -229,7 +229,6 @@ var ImageDialog = Widget.extend({
         this.push(attachment);
         this.$('input.url').val('');
         this.search();
-        this.images = [];
     },
     form_submit: function (event) {
         var self = this;
@@ -342,24 +341,20 @@ var ImageDialog = Widget.extend({
     },
     select_existing: function (e) {
         var $img = $(e.currentTarget);
-        if (!this.options.select_images) {
-            this.$('.o_existing_attachment_cell.o_selected').removeClass("o_selected");
-            $img.closest('.o_existing_attachment_cell').addClass('o_selected');
-        } else {
-            $img.closest('.o_existing_attachment_cell').toggleClass('o_selected');
-        }
         var attachment = _.find(this.records, function (record) { return record.id === $img.data('id'); });
         this.push(attachment);
         this.selected_existing();
     },
     selected_existing: function () {
         var self = this;
+        this.$('.o_existing_attachment_cell.o_selected').removeClass("o_selected");
         var $select = this.$('.o_existing_attachment_cell [data-src]').filter(function () {
             var $img = $(this);
             return !!_.find(self.images, function (v) {
                 return (v.url === $img.data("src") || ($img.data("url") && v.url === $img.data("url")) || v.id === $img.data("id"));
             });
         });
+        $select.closest('.o_existing_attachment_cell').addClass("o_selected");
         return $select;
     },
     try_remove: function (e) {