Add support for setting background and foreground colors of a session's displays using the Xterm Operating System Controls escape sequences.

See the "Operating System Controls" section on this page:
<a href="http://rtfm.etla.org/xterm/ctlseq.html">http://rtfm.etla.org/xterm/ctlseq.html</a>;

Other colors and multiple color sequences are not supported.

xtermcontrol --fg=<color> --bg=<color> now work as expected.

In addition to standard color names ("green","blue","red") etc, Qt also accepts SVG color names:

<a href="http://www.w3.org/TR/SVG/types.html#ColorKeywords">http://www.w3.org/TR/SVG/types.html#ColorKeywords</a>;