[RFR] Add ability to customize icon in Logout button

[steurt]

Goal
For an admin web application I'm building I have applied quite some theming to the default react-admin layout and style. A bit of work, but it is supported quite well and also documented surprisingly well.

I wanted to use a different icon for the logout button. By default it uses the <ExitIcon /> (power symbol). I personally prefer using a different icon to represent the logout action in applications: <ExitToAppIcon/>.

With help of the documentation I found that it was only possible to achieve this by using a completely custom login button component. So I copied the source of the original Logout component and I changed the icon in my application. All good! But not great.

Motivation
The Logout component contains quite a bit of logic. All this logic is now in my custom login button component. The logout button of my app won't benefit from future updates of the react-admin framework to the Logout component due to the usage of a custom component.

Therefore I thought it would be nice to add the option to only change the icon of the default Logout component.

Changes described
I modified the component, described this option in the Theming documentation and also added an entry for the Logout component to the Reference file. I've used it in the simple example locally (not committed) with the following result:

If this PR is accepted and included in a release I can drop the custom login button component and simply configure the icon.