xgettext: Vala: Improve the support of the method .printf().

xgettext: Vala: Improve the support of the method .printf().

* gettext-tools/src/x-vala.c (enum token_type_ty): New enum item
token_type_compound_assign.
(phase3_get): Recognize the compound assignment operators as
token_type_compound_assign instead of token_type_assign.
(extract_balanced): Invoke new_sub_region instead of inheriting_region. Invoke
set_format_flag_on_region instead of iterating through the region. When
encountering an operator, close the inner_region and open a new inner_region.
After 'return', use the passthrough_context_list_iterator, not the
null_context_list_iterator.
* gettext-tools/tests/xgettext-vala-6: Add many more test cases.