embedding: implement cef_string_utf16_cmp()
embedding: begin cef string_list api

cef_string_list_alloc()
embedding: cef_string_list_size()
embedding: cef_string_list_append()
embedding: cef_string_list_value()
embedding: cef_string_list_clear()
embedding: cef_string_list_free()
embedding: cef_string_list_copy()
auto merge of #3679 : zmike/servo/embedding-string_list, r=larsbergstrom

Implements the full cef_string_list API

r+ @larsbergstrom ?