This adds support for creating a `BlendFile` (internally called `Main`),
which is limited to a context.
Temporary data can now be created which can then use
`.libraries.load()` the same as with `bpy.data`.
To prevent errors caused by mixing the temporary ID's with data in
`bpy.data` they are tagged as temporary so they can't be assigned
to properties, however they can be passed as arguments to functions.
Reviewed By: mont29, sybren
Ref D10612
