patch 9.2.0411: tabpanel: no Vim script functions for the tabpanel

patch 9.2.0411: tabpanel: no Vim script functions for the tabpanel

Problem:  tabpanel: no Vim script functions for the tabpanel
Solution: Add tabpanel_getinfo() and tabpanel_scroll()
          (Yasuhiro Matsumoto).

tabpanel_getinfo() returns a dict describing the tabpanel (align,
columns, scrollbar, offset, total, max_offset).

tabpanel_scroll(n) scrolls the tabpanel by n rows (positive for
down, negative for up). With {absolute: 1} the argument is used as
the new absolute offset instead of a delta. The offset is clamped to
the valid range; returns true when it actually changes.

closes: #20056

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>