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/timer.h

@@ -0,0 +1,12 @@
+/**
+ * @file timer.h
+ *
+ * @brief Timer constants
+ *
+ */
+#ifndef TIMER_H
+#define TIMER_H
+
+#define TM_ZERO 0
+
+#endif // TIMER_H