Node Tools: Basic support for data-block inputs

Currently support for data-block inputs is disabled because pointer
properties in operator properties aren't properly handled in Blender
(for more info, see 871c717c6e). This commit brings basic
support for them by storing strings (data-block names) in the operator
properties instead. The main downside of using strings compared other
theoretical solutions is that data-blocks from different library files
can have the same name. This solution won't work well for those cases.
However, it still brings a lot of utility to node tools for a relatively
simple code change.

I investigated two other solutions for this that didn't work out. Using
the recently added enum custom property support didn't work because
the data-block names would still have to be unique. Plus generating an
enum would require a bunch of boilerplate code. Extending the existing
button search code to handle integer session UID backed data-blocks was
much trickier than I expected. The code there is already quite spagetti-
like, and things got out of hand quickly. That's still valid future work
though.

@@ -306,0 +332,4 @@
+    case SOCK_MATERIAL:
+      return ID_MA;
+  }
+  BLI_assert_unreachable();

@@ -323,1 +323,3 @@
-      auto property = bke::idprop::create(identifier, reinterpret_cast<ID *>(value->value));
+      ID *id = reinterpret_cast<ID *>(value->value);
+      if (use_name_for_ids) {
+        return bke::idprop::create(identifier, id ? id->name + 2 : "");