CC Express popup is partly overlapped by other elements

### Expected Behaviour

When opening the CC Express overlay to create a new asset, I expect the overlay not to be obstructed by other page elements, so that I can edit the CC Express project.

### Actual Behaviour

When opening an overlay in the WordPress integration, the sidebar overlaps the CC Express overlay:


Since there is a custom `cc-everywhere-container` element, it is not possible to modify its contents using vanilla JavaScript, jQuery or CSS.
Manually setting the `z-index` for the `.cc-everywhere-root` element to `9991` or higher in the Chrome developer console, brings the editor to the front. This is due to the fact, that the sidebar has a `z-index` of `9990`:


This modification cannot be made using client-side JavaScript or CSS, as stated above.

Depending on the underlying system, it would be great to have a callback – e.g. `onLoad` that passes back the HTML element where the component is mounted or the component itself, so that a developer can modify its appearance to match the website where the dialog is being shown.
Furthermore, more customization options would be great - for example being able to set a max width or max height or specifying an element, where the component will be mounted in (in the case of a WordPress integration, this may be the `.wrap` element).
Otherwise, since it is a popup, the overlay may have a very high `z-index` on its own.

### Reproduce Scenario (including but not limited to)

Will be delivered as soon as there is a working integration to host a public version of the plugin.

#### Steps to Reproduce

* Open the dialog
* See the dialog being obstructed

#### Platform and Version

* Google Chrome 98.0.4758.102
* macOS 12.2.1

#### Sample Code that illustrates the problem

Will be delivered as soon as there is a working integration to host a public version of the plugin.

#### Logs taken while reproducing problem

none

Start with the CC Express overlay in the WordPress integration and reproduce the sidebar overlap in Chrome 98 on macOS 12.2.1; the issue says a public integration is still needed. Done means the overlay is no longer obstructed and the requested mounting or customization behavior has a defined outcome.

Written by the indexing model from the issue text.