xgettext: Add support for .properties files in UTF-8 encoding.

xgettext: Add support for .properties files in UTF-8 encoding.

* autogen.sh (GNULIB_MODULES_TOOLS_FOR_SRC): Add read-file, unistr/u8-check.
* gettext-tools/src/read-properties: Read the file into memory, then find out
about its encoding, then start parsing it.
* gettext-tools/tests/xgettext-properties-2: New file.
* gettext-tools/tests/xgettext-properties-3: New file.
* gettext-tools/tests/xgettext-properties-4: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.
* NEWS: Mention the change.