pdTextBox: Tab order now works!

What a nightmare this was.  I tried every API solution I could find, but
none worked reliably, so I finally resorted to a brute-force search of
all sibling controls, looking for a valid focus target either up or down
for Tab/Shift+Tab.  While not the most elegant solution, it's reliable
and bug-free - so I'll take it.
pdTextBox: finalize focus fixes, add Alt+# support for UTF-32

Surrogate pairs are not supported by the default Alt+# edit box proc.
Even with Windows they are unpredictable; for example, Notepad doesn't
handle them correctly, but WordPad does.  PD now also handles these
correctly, using a custom solution.

Focus should also be sorted out as of this commit.  The last commit had
some unpleasant bugs, but I think I've caught them all now.