More type safety for TUI source window functions

More type safety for TUI source window functions

A few functions can only operate on a source or disassembly window.
This patch adds a bit more type safety to a few of these functions.
This simplifies a subsequent patch.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (tui_clear_source_content)
	(tui_erase_source_content, tui_show_source_content): Change type
	of win_info.
	* tui/tui-winsource.c (tui_clear_source_content)
	(tui_erase_source_content, tui_show_source_content): Change type
	of win_info.
	* tui/tui-win.c (tui_resize_all, tui_adjust_win_heights): Update.
	* tui/tui-source.h (tui_set_source_content_nil): Change type of
	win_info.
	* tui/tui-source.c (tui_set_source_content_nil): Change type of
	win_info.
	* tui/tui-layout.c (show_source_or_disasm_and_command): Update.