gdb/tui: make tui_win_info::title private

This commit builds on this earlier work:

  commit 9fe01a376b2fb096e4836e985ba316ce9dc02399
  Date:   Thu Jun 29 11:26:55 2023 -0600

      Update TUI window title when changed

and makes tui_win_info::title private, renaming to m_title at the same
time.  There's a new tui_win_info::title() member function to provide
read-only access to the title.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>