Refactor ToxWindows

ToxWindow constructor functions now return pointers instead of structs
ToxWindow windows array now holds pointers instead of structs
Refactor some logic in windows.c pertaining to keeping track of active windows
Use uint8_t for window array indexing

src/friendlist.h

@@ -81,7 +81,7 @@ typedef struct {
     ToxicFriend *list;
 } FriendsList;
 
-ToxWindow new_friendlist(void);
+ToxWindow *new_friendlist(void);
 void disable_chatwin(uint32_t f_num);
 int get_friendnum(uint8_t *name);
 int load_blocklist(char *data);