Added splash support (#2)

Combination of quality of life improvements, bugfixes and splash support:

* unzip_file now works with any file, not just a theme

* Made linked list nodes generic so they work for both splashes and themes

* Fixed bug in previous commit with deleting the zip file once it was unzipped

* All zips in /Splashes/ are now extracted, as with /Themes/

* Splashes are all added to the linked list properly

* Splashes can be installed given a path, but only works on Luma. Undecided as to whether or not I'm going to change this.

TODO:

* Automatically change the luma configuration to enable splash screens

* MAYBE add support for other CFWs (if there's an interest). Maybe set a folder to extract splashes to in some kind of user settings?

source/linked_lists.h

@@ -2,11 +2,9 @@
 
 #include "3ds.h"
 
-struct theme_data;
-
 typedef struct
 {
-	struct theme_data *data;
+	void *data;
 	void *next;
 } node;