Support larger terminals automatically

On POSIX systems, determine the main window layout based on the terminal
size at startup; Hardcoded window positioning has been replaced with a
layout which is a function of terminal size. (This is a stepping stone
for a resizable window in-game).

On Windows systems, layout is still determined via 'Viewport size'
options; This patch should be simple to adapt for windows but I am
unable to test on that platform.