Split timer into header and implementation file

timer.c

@@ -0,0 +1,19 @@
+#include <stdbool.h>
+#include "includes/timer.h"
+#include "includes/raygui/raygui.h"
+
+Timer timer_init(double lifetime_secs) {
+    Timer timer;
+    timer.start_time = GetTime();
+    timer.lifetime = lifetime_secs;
+    
+    return timer;
+}
+
+bool timer_done(Timer timer) {
+    return GetTime() - timer.start_time >= timer.lifetime;
+}
+
+double timer_get_elapsed(Timer timer) {
+    return GetTime() - timer.start_time;
+}