[FIX] mass_mailing: prevent click on empty area of drop-down

Currently, when clicking on the empty area during theme selection, the
snippet menu is visible on the top because the drop-down is expanded
which should only happen clicking on the '[data-toggle="dropdown"]'
button for the css and transformation etc to be applied correctly.

This commit fixes the issue by preventing click event and it's
propagation on the whole drop-down, and instead it lets the clicks
being handled only where needed.

taskID-2812312

closes odoo/odoo#88551

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>