Implement floating windows

As of this commit, there's not-yet a way to resize or move floating
windows, but it's possible to create one and focus through all windows.

Floating windows are always above tiled windows and, if floating window
is focused, that window is always above any another floating windows.

Windows have a separate float_{x, y, width, height} to remember their
floating location if they go from float=>tiled=>float again.

src/Config.zig

@@ -330,6 +330,7 @@ fn loadKeybindsChildBlock(config: *Config, parser: *kdl.Parser) !void {
                         .focus_prev_output,
                         .send_to_next_output,
                         .send_to_prev_output,
+                        .toggle_float,
                         .zoom,
                         .reload_config,
                         .toggle_fullscreen,