Switch TagOverlay to use river_shell_surface_v1

This follows the same patterns that Wallpaper and Bar did and makes
TagOverlay use the same manage/render cycle as the rest of the WM.

We also switched to just use a poll timer like river-tag-overlay instead
of using the timerfd. I realized that the Zig stdlib doesn't actually
support timerfds for FreeBSD right now and I don't feel like adding them.

src/XkbBindings.zig

@@ -140,6 +140,7 @@ const XkbBinding = struct {
         switch (xkb_binding.command) {
             .spawn => |cmd| {
                 var child = std.process.Child.init(cmd, utils.gpa);
+                child.env_map = &context.env;
                 _ = child.spawn() catch |err| {
                     log.err("Failed to spawn \"{s}\": {}", .{ cmd[0], err });
                 };