Split mode bits between Term and TermWindow

Split mode bits between Term and TermWindow

Moves the mode bits used by x.c from Term to TermWindow, absorbing
UI/input-related mode bits (visible/focused/numlock) along the way.

This is gradually reducing external references to Term.  Since
TermWindow is already internal to x.c, we add xsetmode() to allow st to
modify window bits in accordance with escape sequences.

IS_SET() is redefined accordingly (term.mode in st.c, win.mode in x.c).

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>