Fix #103761: creating a color attribute doesn't make it active

Originally caused by 6514bb05ea

More cases where the active/default color attributes were not set
correctly:

[1] Using the old Python vertex_colors API (vertex_colors.new)
[2] OBJ importer
[3] Collada importer
[4] Data Transfer layout (both standalone operator and "Generate Data
Layers" from the modifier)

Similar to 101d04f41f.

Brought over from https://archive.blender.org/developer/D16977, see
discussion there why some of the code for data transfer is not for the
genereal attribute API.

@@ -253,0 +258,4 @@
+
+static CustomDataLayer *data_transfer_attributes_color_float_find(const ID *id, const char *name)
+{
+  if (!name) {

@@ -253,0 +262,4 @@
+    return nullptr;
+  }
+  CustomDataLayer *layer = BKE_id_attribute_find(id, name, CD_PROP_COLOR, ATTR_DOMAIN_POINT);
+  if (layer == nullptr) {

@@ -253,0 +284,4 @@
+
+/**
+ * When transfering color attributes, also transfer the active color attribute string.
+ * If a match cant be found, use the first color layer that can be found (to ensure a valid string

@@ -253,0 +256,4 @@
+ * is set).
+ */
+static void data_transfer_mesh_attributes_transfer_active_color_string(
+    Mesh *mesh_dst, Mesh *mesh_src, const eAttrDomainMask mask_domain, const int data_type)