Add DECOM sequence

DECOM sequence allows to the user defines a new home position. The home
position is used as base for all the movement commands except HVP and
VPA. It is important notice than DECSLM moves cursor to absolute position
0,0.
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 st.c |   33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)