a11y: Implement QAccessibleEditableTextInterface

This allows editing the document text by using
the corresponding interfaces on the accessibility
layer, like the AT-SPI EditableText interface on Linux,
e.g. to modify the document using Accerciser [1]
(via the text view in the "Text" section in its
interface view).

[1] https://gitlab.gnome.org/GNOME/accerciser