Blender Git Loki

UI: Improve layout of custom property edit panel

This patch makes the layout of the custom property panel more coherent
with the rest of the property editor interface, makes it less busy,
allows more space for the buttons for the actual properties, and
simplifies editing values of unsupported property types or long arrays.

- Remove the box around each property.
- Use an non-embossed X icon for deleting.
- Use an "edit" icon instead of the text for the meta-data edit operator.
The "gear" icon used for editing isn't ideal here.
- Increase the max array length for drawing the values directly to 8.
- Add an "Edit Property Value" operator for dictionaries or longer arrays.
- Replace the "Library Override" text with an icon.
- Use a proper split factor, the same as the rest of the UI.

Differential Revision: https://developer.blender.org/D12805