Support XTPUSHCOLORS / XTPOPCOLORS

[christianparpart]

It would be really nice to find out if there are actually any apps using these, however.

From ctlseqs.txt

CSI # P
CSI Pm # P
          Push current dynamic- and ANSI-palette colors onto stack
          (XTPUSHCOLORS), xterm.  Parameters (integers in the range 1
          through 10, since the default 0 will push) may be used to
          store the palette into the stack without pushing.

CSI # Q
CSI Pm # Q
          Pop stack to set dynamic- and ANSI-palette colors
          (XTPOPCOLORS), xterm.  Parameters (integers in the range 1
          through 10, since the default 0 will pop) may be used to
          restore the palette from the stack without popping.

CSI # R   Report the current entry on the palette stack, and the number
          of palettes stored on the stack, using the same form as
          XTPOPCOLOR (default = 0) (XTREPORTCOLORS), xterm.