Fixes for -Wwrite-strings

Fixes for -Wwrite-strings

This changes some APIs in compatible ways:
- Some functions now take "const char *" arguments, not "char *"
- Some structs now have "conts char *" members, not "char *"
The changes may cause warnings when compiling with the right warning
flags. You've been warned.

Also adds -Wwrite-strings as a warning flag in configure.ac.

https://bugzilla.gnome.org/show_bug.cgi?id=611692