[FIX] website, *: wait images when setting masonry images wall layout

*: web_editor

During the masonry layout calculation of the images wall snippet, the
image height is used to determine into which column each image is
inserted. Their height is zero until they are actually loaded. Because
of this, the column into which an image is inserted can be wrong.

This becomes more obvious in 16.0 because since [1] the image selection
is lost when moving it within an Image Wall because it is replaced by a
clone when using masonry mode.

This commit makes sure that the images are loaded before taking their
height into account when building the masonry layout.

This involves two changes:

1. By awaiting `wUtils.onceAllImagesLoaded(this.$target)` after the
insertion of each cloned image, we are sure that the reached height of
each column is available before deciding where to insert the next image.

2. Before re-selecting the previously selected image, we need to be
sure that it is loaded. Therefore we keep track of the last masonry
layout operation and await for it. This way, we rely on the await of
the last image as described in point 1.

Additionally, as of 16.0, there is a race condition with
`snippet_option_update`: in some situations, `notify` is called before
`snippet_option_update` is completed, and before the masonry layout is
applied.  To make sure it is completed, the whole notify is run within
the mutex through a `snippet_edition_request` event.

To uphold the stable policy, making `mode` async will only be done in
master.
In stable, a new `_modeWithImageWait` is added which is called only
for the situations that causing an issue, and which enables the fix
of the masonry layout.
In 14.0, it is used when computing the layout from `start` and
`addImages`.
In 16.0, it is also used in `notify` when images are removed or moved
around - because masonry clones the images.
In master, it is always applied.

Additionally, this commit also rounds the computed size of images inside
the masonry layout calculations.

Steps to reproduce:
- Drop an Images Wall.
- Add four images, the first one being taller than the others.

=> The fourth image sometimes appeared below the tall image.

[1]: https://github.com/odoo/odoo/commit/0d43aec24baad6420e0fe150a9c19d33c0b74198

task-2990053

Part-of: odoo/odoo#124608