This is a simple renaming without adaptation.

This rev. is the first part (out of 3) of the refactoring of the
ActionManager and ViewManager layer of the webclient.

The main changes are:
  - there is no ViewManager anymore, its work is now handled by the
    ActionManager itself, but isolated in a specific file ; this
    eases a lot of things, as the breadcrumbs handling for example.
  - the ActionManager code is converted to the new coding principles
    and guidelines ; mainly, children widgets communicate with it by
    triggering events up, and not by function calls anymore.
  - the ActionManager layer is now testable, and a lot of tests have
    already been written.
  - the code in other addons has been adapted consequently.

What's coming next:
  - introduce the 'AbstractAction' Widget, and make client actions
    and view controllers inherit from it ; this widget will
    implement a common API that could be used uniformly by the
    ActionManager (e.g. restore(), canBeLeft(), renderButtons(),
    getTitle()...).
  - move the ControlPanel handling from the ActionManager to the
    AbstractAction.

This commit basically reverts 4974f8edb76e27471bb00c1659c878d0b7a04daf
The fix was unstable (as restoring a feature that was not there
before release, even if originally merged before). This was in
particular breaking saas modals as lots of them are still opened
using 8.0 system... and opening them this way might make them appear
below their own backdrop in the new system the mentioned commit
restored.

We will think about that feature again for a future version.

Commit https://github.com/odoo/odoo/commit/7dcb8fea642858e0bb1d68cea9707cb3c8d4328d
(and https://github.com/odoo/odoo/commit/e64cf3d606bd94f7e0d82ec907ce7d1d66222287)
were supposed to make submodals backdrop appear on top of parent modals.

The feature was however broken by another commit before it was released:
https://github.com/odoo/odoo/commit/f997c128c7295b5e76d7f77d79c4a82898debbcf

As can be seen in https://github.com/scottjehl/Device-Bugs/issues/8 a
combination of "-webkit-overflow-scrolling:touch", "position:relative"
and "overflow:auto" can have unexpected behavior on iOS.

The issue solved here is that writing in a form view inside a modal in
iOS (at least version 10.3) would make the whole form body invisible
until edition is ended.

This commit fixes this by adding a CSS rule which avoid the iOS bug.

opw-774396
closes #20314

Following https://github.com/odoo/odoo/commit/5658e337c3e0385d3d116e34404ef4086b3e94f0:
The modal.less file is now in the common assets so that the
dialogs' style is shared instead of duplicated.

Odoo modals are supposed to be instantiated thanks to the Dialog JS
class. This allows to not duplicate modal structures and to properly
handle multi modals (which bootstrap is not supposed to).

Problem since commit 7dcb8fea642858e0bb1d68cea9707cb3c8d4328d:
The support for inline modals has been broken without providing a
solution for all cases. Indeed, some modals are placed directly inside
form view xml. See an example here:
https://github.com/odoo/odoo/blob/dd0533897e397dfd78ad456fea42a5a12db51246/addons/website_twitter/views/website_twitter_settings_views.xml#L24

While waiting for a new way to instantiate these inline view modals,
this commit restores their deprecated support.

When opening a modal on top of another, we expect the associated
modal-backdrop to be over this another modal (so that it is grayed).
This was not the case as bootstrap forces the modal-backdrop z-index
to a lower value than the modal's one (see fix comments in the changes
below).