Support for custom controls under theme settings

[dziudek]

Needs bigger research:

• if it is possible to load custom component in the fly (potential solution: https://github.com/alexjoverm/v-runtime-template ),
• custom component should load option value, parse it and then display its state,
• before theme settings save, the component should return the new state,
• how we can store custom component in the theme JSON file?
• how to store custom component value in the theme settings?
• check if it is possible to use single component files (with CSS / JS)