Explicitly declare void for all void parameter functions (#628)

Apparently in C, if you have `void test();`, it's completely okay to do
`test(2);`. The function will take in the argument, but just discard it
and throw it away. It's like a trash can, and a rude one at that. If you
declare it like `void test(void);`, this is prevented.

This is not a problem in C++ - doing `void test();` and `test(2);` is
guaranteed to result in a compile error (this also means that right now,
at least in all `.cpp` files, nobody is ever calling a void parameter
function with arguments and having their arguments be thrown away).
However, we may not be using C++ in the future, so I just want to lay
down the precedent that if a function takes in no arguments, you must
explicitly declare it as such.

I would've added `-Wstrict-prototypes`, but it produces an annoying
warning message saying it doesn't work in C++ mode if you're compiling
in C++ mode. So it can be added later.

desktop_version/src/Textbox.h

@@ -8,25 +8,25 @@
 class textboxclass
 {
 public:
-    textboxclass();
+    textboxclass(void);
 
-    void centerx();
+    void centerx(void);
 
-    void centery();
+    void centery(void);
 
-    void adjust();
+    void adjust(void);
 
     void initcol(int rr, int gg, int bb);
 
     void setcol(int rr, int gg, int bb);
 
-    void update();
+    void update(void);
 
-    void remove();
+    void remove(void);
 
-    void removefast();
+    void removefast(void);
 
-    void resize();
+    void resize(void);
 
     void addline(std::string t);
 public: