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/KeyPoll.h

@@ -42,28 +42,28 @@ public:
 
 	int sensitivity;
 
-	int inline getThreshold();
+	int inline getThreshold(void);
 
-	KeyPoll();
+	KeyPoll(void);
 
-	void enabletextentry();
+	void enabletextentry(void);
 
-	void disabletextentry();
+	void disabletextentry(void);
 
-	void Poll();
+	void Poll(void);
 
 	bool isDown(SDL_Keycode key);
 
 	bool isDown(std::vector<SDL_GameControllerButton> buttons);
 	bool isDown(SDL_GameControllerButton button);
-	bool controllerButtonDown();
+	bool controllerButtonDown(void);
 	bool controllerWantsLeft(bool includeVert);
 	bool controllerWantsRight(bool includeVert);
 
 	int leftbutton, rightbutton, middlebutton;
 	int mx, my;
 
-	bool textentry();
+	bool textentry(void);
 	bool pressedbackspace;
 	std::string keybuffer;