[FIX] web_editor: properly set media dialog `onlyImages` and find video

Media dialog was trying to reach the current media's parent node, which
was not possible as it was cloned on instantiation.
To fix this, the `showImageDialog` now passes the `onlyImages` option
upon instantiation, checking the parent's data. It also passes the
media's parent if said parent is a video. This way, neither of these two
things need to be done in media dialog.

closes odoo/odoo#31634

Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>