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/prompt.h

@@ -42,7 +42,7 @@ typedef struct {
 
 extern FriendRequests FrndRequests;
 
-ToxWindow new_prompt(void);
+ToxWindow *new_prompt(void);
 
 void prep_prompt_win(void);
 void prompt_init_statusbar(ToxWindow *self, Tox *m, bool first_time_run);