gladeui: fix a pair of printf security problems

In two cases, gladeui was doing the equivalent of

  printf (var);

instead of the correct (and safe):

  printf ("%s", var);

This was caught by clang's pedantic treatment of non-literal format
strings forcing me to add G_GNUC_PRINTF attributes to functions
operating on format strings (which in turn caught the problem).  Those
changes are also included here.

https://bugzilla.gnome.org/show_bug.cgi?id=720883
glade-editor-property: use literal format strings

Refactor glade_eprop_object_dialog_title() to make use of literal format
strings instead of storing the format string into a variable.

It is my opinion that this change also improves readability of the
function in question by reducing the complexity and redundancy of the
checks.

This fixes the build which was broken on clang due to the use of
-Werror=format-nonliteral.

https://bugzilla.gnome.org/show_bug.cgi?id=720884