[Feature request] OSC 4, 10/11/12/17 support

OSC 10-17 have the ability to get and set the terminal bg/fg/cursor/highlight color. OSC 4 lets you get and set the actual RGB values of the terminal 256-color palette, including the 16 ANSI colors, and could be used to avoid blindly outputting clashing colors to the screen. I'm writing a little tool to set the terminal colors and I keep having to special-case iTerm2 (by checking $TERM_PROGRAM etc) because it has its own (less complicated) way of setting the colors via escape sequences (but not reporting on the current settings).

Here's the spec for these OSC control sequences: https://github.com/JessThrysoee/xtermcontrol/blob/master/doc/ctlseqs.txt#L745.

Thank you, Katherine Crowson

Note1: A newer version of this doc is at http://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h2-Operating-System-Commands (it adds OSC 19).

Note2: If implemented, they should go together with their resetting 104, 110-119 counterparts.

changed milestone to %Review for next version

Note3: In VTE we used to have problems because these escape sequences and the API methods (that is, gnome-terminal's profile preferences) both used to set the same value, kicking out each other. It was solved in https://bugzilla.gnome.org/show_bug.cgi?id=705985 by introducing two levels. For each position in the palette (and special colors like default fg/bg) we remember two colors: the one set by OSC 4/10-19 which is initially empty and is cleared again by 104/110-109, and the API one which is always nonempty. When actually painting the screen, the OSC one takes effect if nonempty, otherwise we fall back to the API. This handles correctly even the situation when the colors are changed via profile settings while an OSC redefinition is active (meaning that the OSC redefinition still remains active at this point).