Add color support for the TUI

[kannibalox]

This uses a subset of the rtorrent-ps color commands and logic to do coloring of the download list and header/footer, in both normal and collapsed view. Color strings work with the basic syntax of <fg attributes> <fg color> [on <bg attributes> <bg color>], with a faux-attribute of bright for 256-color enabled terminals.

Without any color
with a simple config of:

ui.color.title.set="bold bright white on blue"
ui.color.queued.set=blue
ui.color.leeching.set=green
ui.color.odd.set=dim
ui.color.focus.set=reverse
ui.color.footer.set="bold bright white on blue"

Expanded list
Compact list

Fails gracefully when the terminal doesn't support the specified color (note that attributes will still work), e.g.

$ TERM=xterm-old ./src/rtorrent -n -o ui.color.leeching.set=green
rtorrent: green: your terminal only supports 0 colors.

Works correctly in daemon mode, and adds a couple extra checks to avoid unneeded redraw()s

Closes #398