Certain members of AccessText should be public

Certain members of `AccessText` should be public to enable people to write better code by avoiding code copies or usage of reflection.
The members in `AccessText` especially affect custom automation peers for button like controls.

Members that should be public:
- `AccessKeyMarker` => this would avoid the need to hard code "_"
- `RemoveAccessKeyMarker` => this avoid the need to write custom code to achieve the same

Original question/request:
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Currently `AccessText.RemoveAccessKeyMarker` is marked as internal.
To be able to write `AutomationPeer` classes of the same quality as the included ones, `AccessText.RemoveAccessKeyMarker` should be public.

To be honest, i think that all classes/methods used to write controls/automation peers should be public and not internal.

Thinking further about this:
Should we create a list of things that should be public instead of internal?