Earlier, In ERP5Site, we use property 'isIndexable' to have control over indexing and unindexing.
This was usefull specially in case of bootstrap. To have this control, we had a function
`isIndexable` in SQLCatalog which we used to switch for enabling-disabling indexation. (769f9fb2)

Now, we do have ERP5 Catalog, which itself is an ERP5 Object, hence it has an accessor `isIndexable`
of itself which we plan to disable for now(as we don't want circular dependency while indexing
catalog in itself). But as we did have a function `isIndexable` in SQLCatalog which had control
over the indexing, we needed to rename this property for ERP5Site.

Hence, now we use property 'isIndexingRequired' for ERP5Site and use to have overall control
over indexing process.

Please note, we do not intend to change the same property name for any other object. Its just
for ERP5Site.