Some i18n fixes for the TUI

Some i18n fixes for the TUI

The TUI has a few #defines that hold user-visible strings.  As these
are only used in a single spot, this patch removes the defines,
preferring direct use of the string where needed.  Furthermore, now
the strings are wrapped in _(), which is friendlier for i18n purposes.

gdb/ChangeLog
2019-08-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-source.h (struct tui_source_window): Update.
	* tui/tui-regs.c (tui_show_registers): Update.
	* tui/tui-disasm.h (struct tui_disasm_window): Update.
	* tui/tui-data.h (NO_SRC_STRING, NO_DISASSEM_STRING)
	(NO_REGS_STRING): Remove defines.