Enhance flexgrid support

[hhaensel]

Currently, row(), cell() and column() all support flexgrid kwargs size, xs, sm, md, lg, xl to insert respective class names. e.g. col-5, col-md-4.

It would be very convenient to also support all other element functions in StippleUI to support these arguments, however, for a bunch of functions, e.g. btn(), chip() etc. the size argument is already used.

I see two ways out:

• rename the size argument for all functions, possibly keeping size for compatibility for row(), cell() and column()
• replace the size argument only for the functions that define their own size parameter

@essenciary, @AbhimanyuAryan What do you think, and what would be a good name: flex or col or colsize ...

[hhaensel]

Another possible name would be :grid

[hhaensel]

We agreed on :col