refactor: Add kscan sideband behavior driver

* Instead of gpio key behavior trigger, add new kscan driver that
  decorates/wraps a given kscan driver and will invoke basic system
  behavior assigned to a given row + column, without the need for keymap
  mapping in the matrix transform, bypassing keymaps entirely.

app/CMakeLists.txt

@@ -25,11 +25,11 @@ target_sources(app PRIVATE src/stdlib.c)
 target_sources(app PRIVATE src/activity.c)
 target_sources(app PRIVATE src/behavior.c)
 target_sources(app PRIVATE src/kscan.c)
+target_sources_ifdef(CONFIG_ZMK_KSCAN_SIDEBAND_BEHAVIORS app PRIVATE src/kscan_sideband_behaviors.c)
 target_sources(app PRIVATE src/matrix_transform.c)
 target_sources(app PRIVATE src/sensors.c)
 target_sources_ifdef(CONFIG_ZMK_WPM app PRIVATE src/wpm.c)
 target_sources(app PRIVATE src/event_manager.c)
-target_sources_ifdef(CONFIG_ZMK_GPIO_KEY_BEHAVIOR_TRIGGER app PRIVATE src/gpio_key_behavior_trigger.c)
 target_sources_ifdef(CONFIG_ZMK_PM app PRIVATE src/pm.c)
 target_sources_ifdef(CONFIG_ZMK_EXT_POWER app PRIVATE src/ext_power_generic.c)
 target_sources_ifdef(CONFIG_ZMK_GPIO_KEY_WAKEUP_TRIGGER app PRIVATE src/gpio_key_wakeup_trigger.c)

app/Kconfig

@@ -512,6 +512,12 @@ config ZMK_KSCAN_EVENT_QUEUE_SIZE
 
 endif # ZMK_KSCAN
 
+config ZMK_KSCAN_SIDEBAND_BEHAVIORS
+    bool
+    default y
+    depends on DT_HAS_ZMK_KSCAN_SIDEBAND_BEHAVIORS_ENABLED
+    select KSCAN
+
 menu "Logging"
 
 config ZMK_LOGGING_MINIMAL

app/Kconfig.behaviors

@@ -1,11 +1,6 @@
 # Copyright (c) 2023 The ZMK Contributors
 # SPDX-License-Identifier: MIT
 
-config ZMK_GPIO_KEY_BEHAVIOR_TRIGGER
-    bool
-    default y
-    depends on DT_HAS_ZMK_GPIO_KEY_BEHAVIOR_TRIGGER_ENABLED
-
 config ZMK_BEHAVIOR_KEY_TOGGLE
     bool
     default y

app/boards/shields/zmk_uno/boards/nrf52840dk_nrf52840.overlay

@@ -57,10 +57,19 @@ encoder: &qdec0 {
         wakeup-sources = <&wakeup_source>;
     };
 
-    soft_off_behavior_key {
-        compatible = "zmk,gpio-key-behavior-trigger";
-        status = "okay";
-        bindings = <&soft_off>;
-        key = <&button0>;
+    soft_off_direct_kscan: soft_off_direct_kscan {
+        compatible = "zmk,kscan-gpio-direct";
+        input-keys = <&button0>;
     };
+
+    soft_off_sideband_behaviors {
+        compatible = "zmk,kscan-sideband-behaviors";
+        kscan = <&soft_off_direct_kscan>;
+        soft_off {
+            row = <0>;
+            column = <0>;
+            bindings = <&soft_off>;
+        };
+    };
+
 };

app/dts/bindings/kscan/zmk,kscan-sideband-behaviors.yaml

@@ -0,0 +1,29 @@
+# Copyright (c) 2023, The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: |
+  kscan sideband behavior runner. Only basic system behavior should be used,
+  since no keymap processing occurs when using them.
+
+compatible: "zmk,kscan-sideband-behaviors"
+
+include: [kscan.yaml]
+
+properties:
+  kscan:
+    type: phandle
+    required: true
+
+child-binding:
+  description: "A sideband behavior tied to a row/column pair"
+
+  properties:
+    row:
+      type: int
+      required: true
+    column:
+      type: int
+      required: true
+    bindings:
+      type: phandle-array
+      required: true