Add tabs field into Term struct

Add tabs field into Term struct
Tabs stop are simulated in st using a fixed size of 8, always, without be
worried about sequences changing the tab stops. A user can put a tab stop in
each horizontal position of the screen, so we need at least one flag for
each column of the screen. In the same way as dirty flags is used for the
rows, it is used a bool dinamic array.

Signed-off-by: Roberto E. Vargas Caballero <k0ga@shike2.com>
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 st.c |   22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)