Performance Improvements

Pass theme list & its entries around by reference rather than copying them.
Fix bug in async icon loading that caused icons to be loaded multiple times.

Original PR by @LiquidFenrir

include/splashes.h

@@ -31,7 +31,7 @@
 #include "loading.h"
 
 void splash_delete(void);
-void splash_install(Entry_s splash);
+void splash_install(const Entry_s * splash);
 
 void splash_check_installed(void * void_arg);