Style: Fixup instances of void arguments

The style we have here is that functions with no arguments are to have
explicit `void` arguments, even though this doesn't apply in C++,
because it does apply in C.

Unfortunately, some have slipped through the cracks. This commit fixes
them.

desktop_version/src/Textbox.cpp

@@ -159,7 +159,7 @@ void textboxclass::padtowidth(size_t new_w)
     resize();
 }
 
-void textboxclass::centertext()
+void textboxclass::centertext(void)
 {
     padtowidth(w-16);
 }