Change return type of tui_layout_base::adjust_size

Change return type of tui_layout_base::adjust_size

This changes tui_layout_base::adjust_size to return a new enum type.
I broke this out into a separate patch because it simplifies a
subsequent patch.

gdb/ChangeLog
2020-02-22  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.h (enum tui_adjust_result): New.
	(class tui_layout_base) <adjust_size>: Return tui_adjust_result.
	(class tui_layout_window) <adjust_size>: Return
	tui_adjust_result.  Rewrite.
	(class tui_layout_split) <adjust_size>: Return tui_adjust_result.
	* tui/tui-layout.c (tui_layout_split::adjust_size): Update.

Change-Id: I821b48ab06a9b9485875e147bd08a3bc46b900a0