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Preferences: Customize default empty size for collection instances

The old value (1.0) was often too large in practice. When many collection
instances are created, the large empties create a mess in the viewport.

This adds a new preference setting in `Editing -> Objects -> New Objects`
called `Instance Empty Size`.

The value will be used as display size for new empties containing a
collection instance.

Reviewers: Severin

Differential Revision: https://developer.blender.org/D7650