[FIX] web_editor: make crop widget visible again

Crop widget is implemented as div block that is supposed to be displayed on top
of other elements. However, it's not the case for dialogs, because they want to
be on top as well (z-index of dialog is 1050 bootstrap [1])

STEPS:
* Go to Sales Order
* SO
* Schedule an activity
* Use '/' command
* select image
* click image
* crop image

BEFORE: image crop toolbar is hidden behind 'Schedule activity' Pop-up

Fix it by increasing z-index for the crop widget

[1]: https://github.com/odoo/odoo/blob/9102df15cce455205a19fb7b9a0c36186cc51e3a/addons/web/static/lib/bootstrap/dist/css/bootstrap.css#L5524



opw-3391583

closes odoo/odoo#128830

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>