Layout tiled using a width/height fraction

Instead of using container->width/height as both the input and output
of the layout calculation have container->width_fraction/height_fraction
as the share of the parent this container occupies and calculate the
layout based on that. That way the container arrangement can always be
recalculated even if width/height have been altered by things like
fullscreen.

To do this several parts are reworked:

- The vertical and horizontal arrangement code is ajusted to work with
  fractions instead of directly with width/height
- The resize code is then changed to manipulate the fractions when
  working on tiled containers.
- Finally the places that manipulated width/height are adjusted to
  match. The adjusted parts are container split, swap, and the input
  seat code.

It's possible that some parts of the code are now adjusting width and
height only for those to be immediately recalculated. That's harmless
and since non-tiled containers are still sized with width/height
directly it may avoid breaking other corner cases.

Fixes #3547
Fixes #4297

include/sway/tree/container.h

@@ -88,6 +88,10 @@ struct sway_container {
 	double saved_x, saved_y;
 	double saved_width, saved_height;
 
+	// The share of the space of parent container this container occupies
+	double width_fraction;
+	double height_fraction;
+
 	// These are in layout coordinates.
 	double content_x, content_y;
 	int content_width, content_height;