Unicode strings

I've completed the removal of the old STRING class with the USTRING in all
the remaining places (about half had been replaced for a few releases).
This should not have any effect on the operation of the phones.

Adding support for non-ascii character sets on a phone still requires
changes to the .p files to provide the character set to use (defaults to
ascii) , to do this add the keyword 'encoding' to each USTRING that needs to
support it. The lgvx8100.p contains an example.

Simon