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/simple_viewer.c

@@ -62,7 +62,7 @@ static const char* get_next_line(const char *text, struct view_info *info)
     total = 0;
     while(*ptr)
     {
-        unsigned short ch;
+        ucschar_t ch;
         n = ((intptr_t)rb->utf8decode(ptr, &ch) - (intptr_t)ptr);
         if (rb->is_diacritic(ch, NULL))
             w = 0;