Implement title alignment

This adds support for `i3 4.16`'s ability to set the title alignment.
The command is `title_align left|center|right`.

When the title is on the right, marks are moved to the left. Otherwise,
they are on the right.

sway/commands.c

@@ -103,6 +103,7 @@ static struct cmd_handler handlers[] = {
 	{ "smart_borders", cmd_smart_borders },
 	{ "smart_gaps", cmd_smart_gaps },
 	{ "tiling_drag", cmd_tiling_drag },
+	{ "title_align", cmd_title_align },
 	{ "titlebar_border_thickness", cmd_titlebar_border_thickness },
 	{ "titlebar_padding", cmd_titlebar_padding },
 	{ "workspace", cmd_workspace },