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/splashes.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include "minizip/unzip.h"
+#include "linked_lists.h"
+
+Result prepare_splashes(node*);
+Result install_splash(u16 *);