Finish 256 colour support for Irssi

256 colour patch is cleaned up and the remaining cases are made work,
this includes especially Theme support, which was not implemented
before. Changes not related to colours were reverted again, making a
review of the two patches against master easier to follow.

As a byproduct of the Hex-colour code parser, the 24bit colours are
also implemented. Actually using them in the terminal is guarded by a
compile time switch (as well as a run time switch), as it breaks the
existing colour protocol and requires additional storage.

To make a seamless usage, down-conversion is provided for 8 and 16
colours.

Diverging from Tom's approach, the colour protocol is reverted back to
the original one. Unfortunately, the changes required in the Theme
engine will break the API.

For more details, please refer to the patch documentation at either
http://irssi-docs.wikispaces.com/Notes-256-Colour or
https://github.com/shabble/irssi-docs/wiki/Notes-256-Colour

configure.ac

@@ -166,6 +166,15 @@ AC_ARG_ENABLE(dane,
 	fi,
 	want_dane=no)
 
+AC_ARG_ENABLE(true-color,
+[  --enable-true-color     Build with true color support in terminal],
+	if test x$enableval = xno ; then
+		want_truecolor=no
+	else
+		want_truecolor=yes
+	fi,
+	want_truecolor=no)
+
 dnl **
 dnl ** SSL Library checks (OpenSSL)
 dnl **
@@ -643,6 +652,12 @@ if test "x$want_dane" = "xyes"; then
 	fi
 fi
 
+if test "x$want_truecolor" = "xyes" -a "x$want_termcap" != "xyes" -a "x$want_terminfo" = "xyes" ; then
+	AC_DEFINE([TERM_TRUECOLOR], [], [true color support in terminal])
+else
+	want_truecolor=no
+fi
+
 AC_CONFIG_FILES([
 Makefile
 src/Makefile
@@ -761,6 +776,7 @@ echo "Building with SSL support ........ : $have_openssl"
 echo "Building with 64bit DCC support .. : $offt_64bit"
 echo "Building with garbage collector .. : $have_gc"
 echo "Building with DANE support ....... : $have_dane"
+echo "Building with true color support.. : $want_truecolor"
 
 echo
 echo "If there are any problems, read the INSTALL file."