[gdb/tui] Fix wrapping strings

I noticed that after resizing to a narrow window, I got:
...
┌────────────────┐
│                │
│[ No Source Avail
able ]           │
│                │
└────────────────┘
...

Fix this by adding two new functions:
- tui_win_info::display_string (int y, int x, const char *str)
- tui_win_info::display_string (const char *str)
that make sure that borders are not overwritten, which get us instead:
...
┌────────────────┐
│                │
│[ No Source Avai│
│                │
│                │
└────────────────┘
...

Tested on x86_64-linux.