xgettext: Factor out commonly used mixed_string_buffer

xgettext: Factor out commonly used mixed_string_buffer

* x-python.c (init_mixed_string_buffer)
(mixed_string_buffer_append_byte)
(mixed_string_buffer_append_unicode_grow)
(mixed_string_buffer_append_unicode)
(mixed_string_buffer_flush_utf16_surr)
(mixed_string_buffer_flush_curr_buffer)
(mixed_string_buffer_append, mixed_string_buffer_result)
(free_mixed_string_buffer): Move to...
* xgettext.c: ...here.
(mixed_string_buffer_alloc): Rename from init_mixed_string_buffer.
(mixed_string_buffer_append_to_curr_buffer): Rename from
mixed_string_buffer_append_byte.
(mixed_string_buffer_append_to_utf8_buffer): Rename from
mixed_string_buffer_append_unicode.
(mixed_string_buffer_grow_utf8_buffer): Rename from
mixed_string_buffer_append_unicode_grow.
(mixed_string_buffer_append_char): Split from
mixed_string_buffer_append.
(mixed_string_buffer_append_unicode): Split from
mixed_string_buffer_append.
(mixed_string_buffer_done): New function merging
mixed_string_buffer_result and free_mixed_string_buffer.
* xgettext.h (mixed_string_buffer): New struct moved from
x-python.c; add logical_file_name and line_number fields.
(mixed_string_buffer_alloc): New function declaration.
(mixed_string_buffer_append_char): New function declaration.
(mixed_string_buffer_append_unicode): New function declaration.
(mixed_string_buffer_done): New function declaration.
* x-javascript.c (init_mixed_string_buffer)
(mixed_string_buffer_append_byte)
(mixed_string_buffer_append_unicode_grow)
(mixed_string_buffer_append_unicode)
(mixed_string_buffer_flush_utf16_surr)
(mixed_string_buffer_flush_curr_buffer)
(mixed_string_buffer_append, mixed_string_buffer_result)
(free_mixed_string_buffer): Remove.