gdb/tui: rename tui_layout_split:set_weights_from_heights

gdb/tui: rename tui_layout_split:set_weights_from_heights

In a following commit I'm going to add the ability to change the width
of a tui window (when in a horizontal layout).  As a result, some of
the places where we currently hard-code references to height need to
be changed to handle either height, or width, based on whether we are
in a vertical, or horizontal layout.

This commit renames set_weights_from_heights to
set_weights_from_sizes, and makes the function use either the height,
or width as appropriate.

Currently, the only place that we call this function is from the
tui_layout_split::set_height function, in a part of the code we will
only reach for vertical layouts, so the new code is not actually being
used, but, this small change will help make later patches smaller, so
I'm proposing this as a stand alone change.

There should be no user visible changes after this commit.