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
e0b5a20231
