[FIX] web_editor: save target change in ir_ui_view link

Steps to reproduce the issue:
=============================
- Install website and events
- Go to events page and enable editor
- Click on customize and enable sidebar
- enable Follow us option
- Select a social media icon and enable open in a new window option
- Save
- Click on the updated icon, it still opens in the same page => the
  changes aren't saved.

Origin of the issue:
====================
The `target` attribute was ignored when saving and `arch_section`.

Solution
========
Add `target` attribute to the allowed root attrs.

opw-4077657

closes odoo#177269

X-original-commit: a0460d9
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>
Signed-off-by: Mahdi Cheikh Rouhou (macr) <macr@odoo.com>

addons/web_editor/models/ir_ui_view.py

@@ -213,7 +213,7 @@ def _are_archs_equal(self, arch1, arch2):
 
     @api.model
     def _get_allowed_root_attrs(self):
-        return ['style', 'class']
+        return ['style', 'class', 'target']
 
     def replace_arch_section(self, section_xpath, replacement, replace_tail=False):
         # the root of the arch section shouldn't actually be replaced as it's