gdb/tui: add left_boxed_p and right_boxed_p member functions

gdb/tui: add left_boxed_p and right_boxed_p member functions

When I initially saw this code in tui_layout_split::apply, I assumed
that this must be a bug:

    /* Two adjacent boxed windows will share a border, making a bit
       more size available.  */
    if (i > 0
        && m_splits[i - 1].layout->bottom_boxed_p ()
        && m_splits[i].layout->top_boxed_p ())
      ...

After all, the apply might be laying out a horizontal layout, right?
So checking bottom_boxed_p and top_boxed_p is clearly wrong.

Well, it turns on, that due to the implementations of these things,
bottom_boxed_p is equivalent to an imagined right_boxed_p, and
top_boxed_p is equivalent to an imagined left_boxed_p.

In this commit I've renamed both top_boxed_p and bottom_boxed_p to
first_edge_has_border_p and last_edge_has_border_p respectively, and
extended the comments in tui_layout_base to mention that these methods
handle both horizontal and vertical layouts.

Now, hopefully, the code shouldn't look like it only applies for
vertical layouts.

There should be no user visible changes after this commit.