Refactor/main menu (#416)

* Add basic common_ui background gfx functions

* Move game main menu to separate file

* clang format

* tmp commit basic docs

* clang format

* scope vars and cleanup

* Quick consistency update. Finalize.

This branch is based on: https://github.com/GBALATRO/balatro-gba/pull/363
Co-authored-by: emiyl <me@emiyl.com>

* Simplify global struct

* clang-format

* Add clang-format to subdirs

* Move timer definitions into their own file

* clang-format

* Fix action yml

* docs

* Update for PR

* Move game start to a state

* Remove lag switch

* Remove GameVariables pointer for callbacks

* Make game_vars a global

* move declaration of game vars in header

* Fix multiple game_vars

* fix for pr

* remove change_background in update again

* clang format

* Make bgCallbacks table static const

---------

Co-authored-by: emiyl <me@emiyl.com>

include/game_variables.h

@@ -6,6 +6,8 @@
 #ifndef GAME_VARIABLES_H
 #define GAME_VARIABLES_H
 
+#include <tonc.h>
+
 /**
  * @brief A central location for all game variables.
  *
@@ -14,7 +16,8 @@
  */
 typedef struct
 {
-
+    s32 timer;
+    u32 rng_seed;
 } GameVariables;
 
 extern GameVariables g_game_vars;