[FileAPI] FileWriter and read-only files

File objects should have a "readOnly" property, indicating whether write
permission is granted by the user.  Files returned from <input> elements
should, by default, set it.  Constructing a FileWriter using a File with its
readOnly property set should throw an exception.

Later, it would be good to also have an attribute on <input> to allow
opening files for write.  For example, <input type=file file=write> to ask
for write permission (setting readOnly if only read permission was granted,
eg. with an "open read-only" checkbox), and <input type=file file=create> to
also ask the browser to show a "Save As" dialog instead of an "Open" dialog.

-- 
Glenn Maynard