[V2] Make overriding the CSS easier by splitting into multiple libraries

Problem/Motivation

Front-end themes may have their own layout builder element styling, and therefore wish to use gin styling for forms in the off canvas dialog, but not in the layout builder render element.

Overriding gin_lb css from a frontend theme is frustrating due to the pervasive use of the !important modifier in a single css file.

Proposed resolution

Split the relevant gin_lb css into multiple libraries, for example:

• gin_lb/gin_lb_element
• gin_lb/gin_lb_dialog

Frontend themes can then easily override/remove the relevant library. Maybe there is a good case for more than 2 libraries.