Dynamic window resize take 2

The #9600 PR became unmergeable, so I cleaned it up and remade it.

This adds drag-resize functionality to the main game window. Options screen now lists Window Width and Window Height in pixels for SDL builds, has a setting to automatically save window size after drag-resizing to the config, and doesn't require a restart for window size changes to take effect.

There are still some issues with redraw after the window resize, and the function doesn't yet work in the curses build.