[FIX] web_editor: deselect snippet when clicking on empty content

When clicking on the pseudo-block that appears in an empty page, the
snippet icons bounce. However, if a snippet is selected it is not
deselected and the icons are not visible.
This is a problem after the previous commit because if an item is
selected within an "On Hover" mega menu when this pseudo block is
clicked, the mega menu is closed, but the snippet remains selected.

This commit removes the snippet selection before bouncing the icons to
avoid this problem.

Steps to reproduce:
- Add a mega menu.
- Set "Sub Menus: On Hover".
- Select an element inside the mega menu.
- Click on the "DRAG BUILDING BLOCKS HERE" pseudo-block.

=> The mega menu was closed, but its inner element remained selected.

task-2825376

closes odoo/odoo#110258

Signed-off-by: Vray Benjamin (bvr) <bvr@odoo.com>