UX: Add other widgets

Problem/Motivation

Improve CSS variables management by adding new widgets.

Proposed resolution

- reduce textfields size depending on the CSS variables

Example, if the variable is a size, only 2 or 3 characters in size is enough.

- for limited options, use a select list

Remaining tasks

Define CSS variables widget plugin type. Inspired by ✨ UX: Add other widgets than select Active

User interface changes

API changes

Data model changes