Make input width consistent by default

[fzaninotto]

Problem

MUI doesn't defaine a consistent input width, and neither do we. As a consequence, forms mixing various input types don't show a proper alignment by default.

See this story for an illustration. Refs #8796.

Solution

Define a minimum input width in each themes.

Before	After

The story has been updated to let the user test all existing themes, as well as an empty theme (default MUI) and a compat theme (to get the same input widths as in react-admin v4)

[fzaninotto]

After thinking it through, I think the hassle isn't worth the benefit. Users will wonder why setting a custom width doesn't shrink an input. And if they worry about the alignment problem, it's easy enough to solve.

I think we should only document the 2 techniques for sizing inputs:

1. make all inputs fullWidth, and lay them out using a layout component (Grid or Stack)
2. Alter the theme to add a minWidth

[fzaninotto]

So after discussion with the rest of the core team, we won't move forward with a default minWidth for inputs as it would cause too much problems with custom (short) widths.

So I'm closing this PR.