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.h

@@ -32,7 +32,7 @@ public:
 
     void padtowidth(size_t new_w);
 
-    void centertext();
+    void centertext(void);
 public:
     //Fundamentals
     std::vector<std::string> lines;