Allow for Dashicons and base64-encoded data:image/svg+xml URIs when specifying menu icons.

Both of these icons can be colored to match the color scheme, including hover states.
Both are accepted for register_post_type()'s menu_icon argument, and also add_menu_page()'s $icon_url argument.

To use a Dashicon, pass the name of the helper class, e.g. 'dashicons-piechart'.
To use an SVG, pass a valid data URI string starting with 'data:image/svg+xml;base64,'.

props helen.
fixes #25147.

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