Initial implementation of 256 colour support for Irssi

This patch implements some 256 colour support for Irssi up from the
previous 16 colours. Initial parsing of the %x/%X format codes is
implemented and the parser accounts in advances the char* for
that.

The colour attributes are widened from 4 to 8 bit. The colour protocol
is changed to a new format. Some pointers to remaining work are
written in the comment in textbuffer.h.

Note that Irssi already does support requesting 256 colours from the
terminal in the original source code, so this part did not have to be
touched.
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-color or
https://github.com/shabble/irssi-docs/wiki/Notes-256-Colour