xbase: use a diff font for the help text and cleanup text

Signed-off-by: David Aguilar <davvid@gmail.com>
xbase: refactor to make the item class simpler

Replace the hard-coded indexes with a runtime computation.  Move the
item.new_combo() to the parent class' decorate_item() function, which
reduces coupling.  We no longer need item.parent.

Signed-off-by: David Aguilar <davvid@gmail.com>
xbase: add keyboard shortcuts for pick, reword, edit, fixup, squash

1, 2, 3, 4, 5
p, r, e, f, s

Signed-off-by: David Aguilar <davvid@gmail.com>