Rework Container spacing UI

[JohnPixle]

@lushkant As discussed on Slack we will need to make some changes here Please review and let me know if you spot any problems / issues.

Notes / changes:

• [x] Users will not be able to add custom padding presets anymore
• [x] If they have created any custom ones with random hashes, then it will be preserved in order not to break their existing layouts
• [x] We will offer them 14 predefined slots Naming will continue as ang_container_padding_24
• [x] Slots 1-8 are free. We shall have 3 tabs with 8 slots each
• [x] Presets/slots appear should only appear at the spacing dropdown only if they are populated. Blank labels/slots should not appear at the dropdown (of the container widgets)
• [x] If the users click on the "reset labels and values" button, then any "random hash" variable they might have created will be deleted. Possible?

Figma mockup is coming soon

[JohnPixle]

Keep in mind that at the containers panel

• [x] when you switch from 2nd tab to the 3rd, the CTA "jumps some pixels". You'll need to see in action

Think we can fix that

[JohnPixle]

We have also have #540 now to take care of.