Add layer shell support and fix floating windows

Now, I actually save the river-layer-shell-v1 and keep track of the
non-exclusive area. The layout calculation uses the usable area instead
of the entire output's geometry.

I removed boundary clamping for the floating windows because it was a
bit janky when hitting the edges. I'll probably add it back at some
point. I also made windows default to 75% of the usable area instead of
keeping their tiled size so that maximized windows look decent when
floating for the first time. Finally, since I removed the clamping, I
added a center_float keybind to center a floating window. If you're
cycling through focused windows and one isn't on the screen, you can use
the center_float bind to get the window visible again.

Replaced all divTrunc with divFloor to be consistent. I think they
should all be positive, anyways, so they'd be the same, but I like just
having one.

src/Config.zig

@@ -506,6 +506,7 @@ fn loadKeybindsChildBlock(config: *Config, parser: *kdl.Parser, hostname: ?[]con
                         .decrement_primary_count,
                         .swap_next,
                         .swap_prev,
+                        .center_float,
                         => |cmd| {
                             // None of these have arguments, just create the union and give it back
                             break :sw @unionInit(XkbBindings.Command, @tagName(cmd), {});