Make drag_icon a persistent scene node

If there is no current drag icon, this node will be empty, but we now
have `drag_icon != NULL` as an invariant.  This allows us to eliminate a
conditional, since there's no harm in moving an empty node's coordinates
around with the pointer.

ΔSLOC: -1

properly destroy scene

ΔSLOC: +1

Unify signal handling under wl_event_loop

Merge our signal handlers into a single function and let Wayland deal
with all the struct sigaction stuff.

ΔSLOC: -3

Separate drag icon from layers array and Lyr enum

If we treat the drag icon as distinct from other layers (it doesn't have
contents that are interactive, focusable, etc.), then we can iterate
over layers meaningfully with a simple for loop.

ΔSLOC: -8

Don't bother with ignoring SIGCHLD

It added complexity, especially with the differences in behavior between
handled and ignored signals across an exec().

Reset ignored signal handler in spawn() as well

fix startup_cmd SIGCHLD handler

Ignored handlers are not reset by exec() calls

Stray whitespace fixes

activate lock surface in updatemons

Fixes an issue when swaylock does not receive input after turn off and then
turn on the outputs

send frame done even if output commit fails

Bug: https://github.com/djpohly/dwl/issues/420
Fixes: https://github.com/djpohly/dwl/issues/353

Set XCURSOR_SIZE

destroy old client popups when focusing another client

Closes: https://github.com/djpohly/dwl/issues/408

Prevent using a wlr_layer_surface after destroying it, due to no available outputs

correctly check if a scene node is enabled

checking only wlr_scene_node.enabled may result in a false positive because it
does not consider if its ancestors are enabled as well.

remove unneeded check of `m` in arrange()

arrange must never be called with a NULL argument

Use uint32_t for ui Arg

use fixed-size type for client tags

while the size of `int` in most compilers is 32-bits, the size of int and all
other integer types are implementation defined, so make sure we can use up to
32-bits

remove tag labels from dwl

Tag labels are not used in dwl. Only the number of tags is important.
Tag labels should be defined for each tag in whatever status bar is
used.

display clients count in monocle symbol

- Replicate missing functionality from dwl to display the client count in monocle mode
  - Add ltsymbol field to Monitor struct
  - Display client count in monocle mode when greater than zero
- Tested with somebar and dwlb

Missing apostrophe