GPv3: RNA property to set active layer

Use "active_index" RNA property to change active layer. This will allow
to sen updates to other regions when active layer is changed and also
registers undo step.

Logic of this property is similar to legay GP.
Right now GPv3 is using `tree_row_click_fn/on_activate` function to change
the active layer.

@@ -117,0 +117,4 @@
+static int rna_Grease_Pencil_active_layer_index_get(PointerRNA *ptr)
+{
+  GreasePencil *grease_pencil = rna_grease_pencil(ptr);
+  int val = grease_pencil->layers().first_index(grease_pencil->get_active_layer());

@@ -117,0 +118,4 @@
+{
+  GreasePencil *grease_pencil = rna_grease_pencil(ptr);
+  int val = grease_pencil->layers().first_index(grease_pencil->get_active_layer());
+  printf("something get");

@@ -117,0 +126,4 @@
+{
+  using namespace blender::bke::greasepencil;
+  GreasePencil *grease_pencil = rna_grease_pencil(ptr);
+  blender::Span<const Layer *> layers = grease_pencil->layers();

@@ -117,0 +118,4 @@
+{
+  GreasePencil *grease_pencil = rna_grease_pencil(ptr);
+
+  if (grease_pencil->get_active_layer() == nullptr) {

@@ -189,6 +189,7 @@ class AbstractTreeViewItem : public AbstractViewItem, public TreeViewItemContain
   virtual ~AbstractTreeViewItem() = default;
 
   virtual void build_row(uiLayout &row) = 0;
+  virtual void add_treerow_button(uiBlock &block);

@@ -170,0 +191,4 @@
+    UI_but_flag_enable(view_item_but_, UI_BUT_UNDO);
+
+    view_item_but_->view_item = reinterpret_cast<uiViewItemHandle *>(this);
+    view_item_but_->rnaprop = RNA_struct_find_property(&layer_ptr, "active_index");

@@ -117,0 +129,4 @@
+{
+  using namespace blender::bke::greasepencil;
+  GreasePencil *grease_pencil = rna_grease_pencil(ptr);
+  const Layer *layer = grease_pencil->layers()[value];

@@ -117,0 +131,4 @@
+  GreasePencil *grease_pencil = rna_grease_pencil(ptr);
+  const Layer *layer = grease_pencil->layers()[value];
+  grease_pencil->set_active_layer(layer);
+  WM_main_add_notifier(NC_GPENCIL | NA_EDITED, NULL);

@@ -184,2 +187,4 @@
+  void on_activate(bContext &C) override
   {
     this->grease_pencil_.set_active_layer(&layer_);
+    WM_event_add_notifier(&C, NC_GPENCIL | ND_DATA | NA_SELECTED, &(this->grease_pencil_));