Add horizontal splitting to TUI layout

Add horizontal splitting to TUI layout

This changes the TUI layout engine to add horizontal splitting.  Now,
windows can be side-by-side.

A horizontal split is defined using the "-horizontal" parameter to
"tui new-layout".

This also adds the first "winheight" test to the test suite.  One open
question is whether we want a new "winwidth" command, now that
horizontal layouts are possible.  This is easily done using the
generic layout code.

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

	PR tui/17850:
	* tui/tui-win.c (tui_gen_win_info::max_width): New method.
	* tui/tui-layout.h (class tui_layout_base) <get_sizes>: Add
	"height" argument.
	(class tui_layout_window) <get_sizes>: Likewise.
	(class tui_layout_split) <tui_layout_split>: Add "vertical"
	argument.
	<get_sizes>: Add "height" argument.
	<m_vertical>: New field.
	* tui/tui-layout.c (tui_layout_split::clone): Update.
	(tui_layout_split::get_sizes): Add "height" argument.
	(tui_layout_split::adjust_size, tui_layout_split::apply): Update.
	(tui_new_layout_command): Parse "-horizontal".
	(_initialize_tui_layout): Update help string.
	(tui_layout_split::specification): Add "-horizontal" when needed.
	* tui/tui-layout.c (tui_layout_window::get_sizes): Add "height"
	argument.
	* tui/tui-data.h (struct tui_gen_win_info) <max_width, min_width>:
	New methods.

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

	PR tui/17850:
	* gdb.texinfo (TUI Commands): Document horizontal layouts.

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

	PR tui/17850:
	* gdb.tui/new-layout.exp: Add horizontal layout and winheight
	tests.

Change-Id: I38b35e504f34698578af86686be03c0fefd954ae