Save text box centering state

This is another piece of state that needs to be kept and re-played when
switching language, because a different language could change the
dimensions of the text box, which affects how it's centered.

Also, to make sure that crewmate positions override any text centering,
the scriptclass variables textx and texty should be reset in the
position and customposition commands.

desktop_version/src/Textbox.cpp

@@ -28,6 +28,9 @@ textboxclass::textboxclass(int gap)
 
     large = false;
 
+    should_centerx = false;
+    should_centery = false;
+
     print_flags = PR_FONT_LEVEL;
     fill_buttons = false;
 
@@ -72,6 +75,14 @@ void textboxclass::adjust(void)
 {
     resize();
     repositionfromcrewmate();
+    if (should_centerx)
+    {
+        centerx();
+    }
+    if (should_centery)
+    {
+        centery();
+    }
     if (xp < 10) xp = 10;
     if (yp < 10) yp = 10;
     if (xp + w > 310) xp = 310 - w;