gdb/tui: rename tui_layout_base::adjust_size to ::set_height

Rename tui_layout_base::adjust_size to tui_layout_base::set_height,
the new name more accurately reflects what this member function does,
and makes it easier for a later commit to add a new
tui_layout_base::set_width member function.

There should be no user visible changes after this commit.