GPv3: Frame delete operator (edit mode).

This operator is a combination of the delete operator's `frame` mode and the `active_frames_delete_all` operator.

This also add the delete menu and key binds.

@@ -560,0 +609,4 @@
+{
+  PropertyRNA *prop;
+
+  static const EnumPropertyItem prop_greasepencil_delete_types[] = {

@@ -560,0 +562,4 @@
+/** \name Delete Frame Operator
+ * \{ */
+
+enum eGREASEPENCIL_DeleteFrameMode {

@@ -560,0 +591,4 @@
+  GreasePencil &grease_pencil = *static_cast<GreasePencil *>(object->data);
+  const int current_frame = scene->r.cfra;
+
+  eGREASEPENCIL_DeleteFrameMode mode = eGREASEPENCIL_DeleteFrameMode(

@@ -560,0 +595,4 @@
+      RNA_enum_get(op->ptr, "type"));
+
+  bool changed = false;
+  if (mode == GREASEPENCIL_DELETEFRAMEOP_ACTIVE_FRAME) {

@@ -560,0 +598,4 @@
+  if (mode == GREASEPENCIL_DELETEFRAMEOP_ACTIVE_FRAME) {
+    bke::greasepencil::Layer &layer = *grease_pencil.get_active_layer_for_write();
+    if (layer.is_editable()) {
+      changed |= layer.remove_frame(layer.frame_key_at(current_frame));

@@ -560,0 +604,4 @@
+  else if (mode == GREASEPENCIL_DELETEFRAMEOP_ALL_FRAMES) {
+    for (bke::greasepencil::Layer *layer : grease_pencil.layers_for_write()) {
+      if (layer->is_editable()) {
+        changed |= layer->remove_frame(layer->frame_key_at(current_frame));

@@ -561,0 +605,4 @@
+    DEG_id_tag_update(&grease_pencil.id, ID_RECALC_GEOMETRY);
+    WM_event_add_notifier(C, NC_GEOM | ND_DATA | NA_EDITED, &grease_pencil);
+  }
+  else {