Blender Git Loki

Cleanup: remove `exec()` calls from custom property editor

Remove `exec()` and `eval()` calls from `WM_OT_properties_edit` where
possible. This not only results in simpler, cleaner code, but also
removes the necessity for `repr(value)` to evaluate to a Python
expression that in turn evaluates to `value` again.

No functional changes.

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D8658