Introduce the tui_gen_win_info::min_height method

Introduce the tui_gen_win_info::min_height method

This introduces a new method, tui_gen_win_info::min_height, to fetch
the minimum height of a window.  This is used in the subsequent
unified layout patch.

gdb/ChangeLog
2019-12-11  Tom Tromey  <tom@tromey.com>

	* tui/tui-stack.h (struct tui_locator_window) <min_height>:
	Implement.
	* tui/tui-regs.h (struct tui_data_item_window) <min_height>:
	Implement.
	* tui/tui-data.h (struct tui_gen_win_info) <min_height>: New
	method.
	(struct tui_win_info) <min_height>: Implement.

Change-Id: Id33baffdf041fde072e15c1ff89b75f8b8118adb