Allocate new strings as return values from text interface

From: Daniel Drake <d.drake@mmm.com>

Like most GTK+ functions which return strings, users are expected to free
the return value from function calls such as atk_text_get_text().

The current functions are just returning the result of PyString_AsString but
the docs make it pretty clear that this refers to an internal buffer which
should not be touched.

Index: papi-0.0.3/src/papi_atktextiface.c
===================================================================
--- papi-0.0.3.orig/src/papi_atktextiface.c
+++ papi-0.0.3/src/papi_atktextiface.c
@@ -309,7 +309,7 @@ _text_get_text (AtkText *text, gint star
 
     retval = PyString_AsString (result);
     Py_DECREF (result);
-    return retval;
+    return g_strdup (retval);
 }
 
 /**
@@ -358,7 +358,7 @@ _text_get_text_after_offset (AtkText *te
 
     retval = PyString_AsString (result);
     Py_DECREF (result);
-    return retval;
+    return g_strdup (retval);
 }
 
 /**
@@ -383,7 +383,7 @@ _text_get_text_at_offset (AtkText *text,
 
     retval = PyString_AsString (result);
     Py_DECREF (result);
-    return retval;
+    return g_strdup (retval);
 }
 
 /**
@@ -409,7 +409,7 @@ _text_get_text_before_offset (AtkText *t
 
     retval = PyString_AsString (result);
     Py_DECREF (result);
-    return retval;
+    return g_strdup (retval);
 }
 
 /**
@@ -693,7 +693,7 @@ _text_get_selection (AtkText *text, gint
             Py_DECREF (val);
         }
     Py_DECREF (result);
-    return retval;
+    return g_strdup (retval);
 }
 
 /**