Use less error prone string list, map or multimap pointer types

Use pointers to incomplete struct types. If an argument of a not suitable
type is passed to a function of any string collection,
C compiler will warn:
  warning: passing argument 1 of <func> from incompatible pointer type
C++ compiler will raise an error, for example:
  error: cannot convert 'cef_string_list_t' ... to 'cef_string_multimap_t'

With the previously used `void*` pointer types, such errors in a code were not
diagnosed.

libcef/common/string_multimap_impl.cc

@@ -13,7 +13,7 @@ using StringMultimap = std::multimap<CefString, CefString>;
 }  // namespace
 
 CEF_EXPORT cef_string_multimap_t cef_string_multimap_alloc() {
-  return new StringMultimap;
+  return reinterpret_cast<cef_string_multimap_t>(new StringMultimap);
 }
 
 CEF_EXPORT size_t cef_string_multimap_size(cef_string_multimap_t map) {