Basic Alembic support

All in all, this patch adds an Alembic importer, an Alembic exporter,
and a new CacheFile data block which, for now, wraps around an Alembic
archive. This data block is made available through a new modifier ("Mesh
Sequence Cache") as well as a new constraint ("Transform Cache") to
somewhat properly support respectively geometric and transformation data
streaming from alembic caches.

A more in-depth documentation is to be found on the wiki, as well as a
 guide to compile alembic: https://wiki.blender.org/index.php/
User:Kevindietrich/AlembicBasicIo.

Many thanks to everyone involved in this little project, and huge shout
out to "cgstrive" for the thorough testings with Maya, 3ds Max, Houdini
and Realflow as well as @fjuhec, @jensverwiebe and @jasperge for the
custom builds and compile fixes.

Reviewers: sergey, campbellbarton, mont29

Reviewed By: sergey, campbellbarton, mont29

Differential Revision: https://developer.blender.org/D2060

release/scripts/startup/bl_ui/properties_data_modifier.py

@@ -181,6 +181,9 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
         layout.prop(md, "cache_format")
         layout.prop(md, "filepath")
 
+        if md.cache_format == 'ABC':
+            layout.prop(md, "sub_object")
+
         layout.label(text="Evaluation:")
         layout.prop(md, "factor", slider=True)
         layout.prop(md, "deform_mode")
@@ -215,6 +218,22 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
         row = split.row()
         row.prop(md, "flip_axis")
 
+    def MESH_SEQUENCE_CACHE(self, layout, ob, md):
+        layout.label(text="Cache File Properties:")
+        box = layout.box()
+        box.template_cache_file(md, "cache_file")
+
+        cache_file = md.cache_file
+
+        layout.label(text="Modifier Properties:")
+        box = layout.box()
+
+        if cache_file is not None:
+            box.prop_search(md, "object_path", cache_file, "object_paths")
+
+        if ob.type == 'MESH':
+            box.row().prop(md, "read_data")
+
     def CAST(self, layout, ob, md):
         split = layout.split(percentage=0.25)