`Wait Until Element Is Enabled` should also check `readonly` status

[pekkaklarck]

Element Should (Not) Be Enabled validates that element is enabled and not read-only, but Wait Until Element Is Enabled only checks is the element enabled. For consistency reasons we should change the latter to also validate read-only status.

At the moment Element Should (Not) Be Enabled also checks is the element type is 'input', 'select', 'textarea', 'button' or 'option'. That's a bad idea because it ignores HTML5 input elements (see also #546). Instead of trying to make the list of valid input elements perfect, I think it's better to not do any type checking.