[FIX] web_editor: fix parent element selection

When a block is removed, the parent will be activated.
If the parent element is empty and removable:

1) It will be removed too.
2) In '_activateSnippet' method, we look for the right element to
   be selected using the block's parent (removed from the DOM...).

The goal of this commit is to fix this behaviour by computing the
selected element before parent removal.

task-2431506

closes odoo/odoo#65428

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>