Rework of alertpanel default button focus handling.

Rework of alertpanel default button focus handling.

Instead of using "+" prefix with button labels and/or
the default_value parameter, we introduce "focus"
parameter, which will point to first, second or third
button.

Advantages:
- we got rid of the "+" prefix, which won't work with GTK3
  with GTK3
- instead of having two ways to indicate which button
  should have focus, which can be in conflict, and
  are not very straightforward, there is just one,
  arguably more intuitive
- some small memory leaks fixed along the way
  (using g_strconcat() directly as function argument)