core: add locale aware formatting function to std::string

We had locale aware formatting functions that generated QStrings.
Create an alternative that creates std::string, since we want that
in the core.

This commit is unfortunate for two reasons:
- The function is called "casprintf()" for analogy with the QString
  version. However, the non locale aware function is called
  "format_string_std()" for analogy with "format_string()".
  Ultimately these names should be unified. Probably, once there
  are no membuffer users left.
- This does UTF-16->UTF-8->UTF-16 roundtrips. The core formatting
  functions should render UTF-8 and only convert to UTF-16, in
  the UI layer.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/format.h

@@ -11,6 +11,7 @@
 #include <QString>
 __printf(1, 2) QString qasprintf_loc(const char *cformat, ...);
 __printf(1, 0) QString vqasprintf_loc(const char *cformat, va_list ap);
+__printf(1, 2) std::string casprintf_loc(const char *cformat, ...);
 #endif
 
 #ifdef __cplusplus