Asset Packs: Optimize icon loader (#164)

* Original pointer can be const

* Back to const icons

* Missed this one

* Simpler string alloc

* Single allocation and header struct for static icons

* Shared allocation and meta struct for animated icons

* Only try to load if dir exists

* Restructure momentum lib

* Use some internal headers

* Swap icons at draw

* Properly init and free, no more original in icon struct

lib/momentum/asset_packs.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <gui/canvas.h>
+#include <stdint.h>
+
+#define ASSET_PACKS_PATH EXT_PATH("asset_packs")
+
+void asset_packs_init(void);
+void asset_packs_free(void);