Simplify TUI boxing
    
    In the TUI, whether or not a window can be boxed is a property of the
    window's type.  This adds a can_box method to the window classes, and
    changes tui_make_window to defer to this, removing the "box_it"
    paramter.  This also lets us remove "enum tui_box", as it is no longer
    used.
    
    gdb/ChangeLog
    2019-08-15  Tom Tromey  <tom@tromey.com>
    
    	* tui/tui-wingeneral.h (tui_make_window): Update.
    	* tui/tui-wingeneral.c (tui_make_window): Remove "box_it"
    	parameter.
    	(tui_gen_win_info::make_visible): Update.
    	* tui/tui-regs.c (tui_data_window::display_registers_from):
    	Update.
    	* tui/tui-layout.c (show_source_disasm_command)
    	(show_source_or_disasm_and_command): Update.
    	* tui/tui-data.h (struct tui_gen_win_info) <can_box>: New method.
    	(enum tui_box): Remove.
    	(struct tui_win_info) <can_box>: New method.
    	* tui/tui-command.h (struct tui_cmd_window) <can_box>: New
    	method.