Add contenteditable to the list of attributes shared by all MathML elements

Since contenteditable (HTML spec) is a valid attribute for any HTML element, it make sense to add it to the list of attributes shared by all MathML elements (MathML spec).

I do not assume any specific behavior for the UA when the contenteditable is present. The only requirement is that the attribute be valid and available in the DOM. From this point of view, this has a null impact on any implementation of MathML. For example, the current implementation of MathML on Firefox allows a clumsy edition of formulas when contenteditable is inherited as true.

The attribute contenteditable can be used a as convenient way to express that some parts of a formula are (or are not) editable. This is an HTML5 friendly way to allow blanks inside a Math formula (for example, it is easy to imagine a fill-the-blank math questionnaire), respect to an alternative solution based on an maction.

Start by reading the MathML shared-attributes section linked in the issue and compare it with the HTML contenteditable definition. Locate the corresponding shared-attribute list in the MathML4 editors draft, add contenteditable there, and confirm the attribute is listed as valid and available in the DOM.