Polish classes used in dropdown

[hmpf]

I also don't like how "jumpy" the width of the dropdown multiselect field becomes, we need a separate issue to fix it. We need both a minimum width and some transitions to smooth out the changes in width.

Originally posted by @podliashanyk in #1118 (comment)

[hmpf]

The height in the widget is also hardcoded, to fit in the filter box.

I have an idea to make a widget mixin that allows fixed and optional classes. So a <select> always has "select selecte-bordered" but which "min-h*" and "min-w*" can be set when instantiating.