QoL Improvements: Better notices

[diggeddy]

Description

An issue to list and track QoL improvements that can be made through better notices in the UI

• [ ] Show text values in inspector controls unit picker. Closes: #910
• [ ] Show notice when adding pseudo element background that nested blocks may require z-index Closes #1111
• [ ] Improve Query Loop notice when post type has no published posts Closes #1096
• [ ] Add notices when fixed unit values are added to width, height and max height Closes #1112
• [ ] Query Loop inform user Posts per Page must have a value Closes #680