GPv3: Conversion: Add some support for GPData animation itself.

Handling animation of GPv3 in itself is relatively straightforward, it's
mainly a matter of duplicating animdata into the new GreasePencil ID.

In case some propoerties need to be remapped, this will be done in a
similar way as e.g. GP object's modifiers animation for Object-level
animation.

The complex and ugly part of this PR is in the need to move animation
from GPdata to Object level for some properties. This PR tackles the
'layer adjustments to modifiers' aspect (i.e. adjustments on tint and
thickness).

Known limitations currently with these GPData to Object animation:
* NLA is not supported (i.e. if an NLA in legacy GP data controls these
  adjustments animations, it won't be converted to Object-level NLA to
  control matching modifiers settings).
* Drivers targets are not handled either, i.e. in case a driver is using
  data from legacy GPdata as input, these will be left as-is (this is
  true for all anim handling currently).
* There is no adjustments of values for animation (e.g. the thickness
  adjustment values would need to be devided by 2000).

Most of these limitations can be addressed at some point, depending on
how critical they are to support. This would have a cost (in time and
code complexity) though.

@@ -60,0 +61,4 @@
+/* Utils to move a list of fcurves from one container (Action or Drivers) to another. */
+static void legacy_fcurves_move(ListBase &fcurves_dst,
+                                ListBase &fcurves_src,
+                                blender::Vector<FCurve *> fcurves)

@@ -502,0 +534,4 @@
+   * Note that currently, Actions IDs are not duplicated. They may be needed ultimately, but for
+   * the time being, assuming invalid fcurves/drivers are fine here. */
+  AnimData *gpd_animdata = BKE_animdata_from_id(&gpd.id);
+  if (gpd_animdata) {

@@ -627,0 +727,4 @@
+          return false;
+        }
+        StringRefNull rna_path = fcurve.rna_path;
+        for (auto prop_path : fcurve_tint_valid_prop_paths) {

@@ -643,0 +786,4 @@
+            return false;
+          }
+          StringRefNull rna_path = fcurve.rna_path;
+          for (auto prop_path : fcurve_tint_valid_prop_paths) {

@@ -675,0 +862,4 @@
+            return false;
+          }
+          StringRefNull rna_path = fcurve.rna_path;
+          for (auto prop_path : fcurve_thickness_valid_prop_paths) {