internals: Support characters beyond the first unicode plane (WIP)

We used 16-bit variables to store the 'character code' everywhere but
this won't let us represent anything beyond U+FFFF.

This patch changes those variables to a custom type that can be 32 or 16
bits depending on the build, and adjusts numerous internal APIs and
datastructures to match.  This includes:

 * utf8decode() and friends
 * on-screen keyboard
 * font manipulation, caching, rendering, and generation
 * VFAT code parses and generates utf16 dirents
 * WIN32 simulator reads and writes utf16 filenames

Note that this patch doesn't _enable_ >16bit unicode support; a followup
patch will turn that on for appropriate targets.

Known bugs:

  * Native players in 32-bit unicode mode generate mangled filename
    entries if they include UTF16 surrogate codepoints.  Root cause
    is unclear, and may reside in core dircache code.

Needs testing on:

 * windows simulator (16bit+32bit)

Change-Id: I193a00fe2a11a4181ddc82df2d71be52bf00b6e6

apps/plugins/lib/kbd_helper.h

@@ -1,10 +1,10 @@
 /***************************************************************************
- *             __________               __   ___.                  
- *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___  
- *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /  
- *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <   
- *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \  
- *                     \/            \/     \/    \/            \/ 
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
  * $Id$
  *
  * Copyright (C) 2020 William Wilgus
@@ -22,6 +22,6 @@
 #define KBD_HELPER_H
 
 /* create a custom keyboard layout for kbd_input */
-int kbd_create_layout(const char *layout, unsigned short *buf, int bufsz);
+int kbd_create_layout(const char *layout, ucschar_t *buf, int bufsz);
 
 #endif /* KBD_HELPER_H */