Initial basic functionality commit

What this commit adds:
* Ability to apply a theme, given a folder
* Extract all .zip files in /Themes/ into a relevant folder
* Get all theme metadata from info.smdh
* Proper unicode handling

What's still needed for initial release:
* Theme shuffling
* GUI

source/unicode.h

@@ -0,0 +1,10 @@
+#pragma once
+
+void wtoa(char*, u16*);
+ssize_t atow(u16*, char*);
+ssize_t trim_extension(u16*, u16*);
+bool strip_unicode(u16*, u16*, ssize_t);
+u16 *strucpy(u16*, const u16*);
+ssize_t strulen(const u16*);
+u16 *strucat(u16*, const u16*);
+void printu(u16*);