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