xgettext: In language Tcl, support \x, \u, \U escapes as specified in Tcl 8.6.

xgettext: In language Tcl, support \x, \u, \U escapes as specified in Tcl 8.6.

* gettext-tools/src/x-tcl.c: Update comments.
(phase1_pushback): Increase size to 5.
(do_getc_escaped): For \x, parse only up to 2 hexadecimal characters.
Handle '\U'.
(do_getc_escaped_low_surrogate): New function.
(accumulate_word): After reading a high surrogate, see if it is followed by a
low surrogate.
* gettext-tools/tests/xgettext-tcl-4: Change expected outcome for \x. Add test
cases for \u with surrogates and for \U.
* gettext-tools/tests/xgettext-tcl-5: Add more test cases.
* NEWS: Mention the change.