[ExperienceStudio] Grid container: explicit column width

* With the current grid container I can select a column amount (1 - 12). But all columns are equally sized.
* I am not able to explicitly say how "wide" a column should be - e.g. two columns: `25% | 75%`
* This could e.g. be needed when building the Sidebar layout (Listing and Category navigation) with the grid.

Use this task also to evaluate how more explicit columns could work and clarify some open questions.
* Do we work with preset percentages?
* Do we allow custom px values?
* Should this be a thing at all or instead be determined by the children element of the grid?