Fix class names for mixin styles

Styles created via assign() did not get new class names
because the resulting object already had 'key' and 'parent'
properties so they were not added by 'style.create()'

Use style.mixin() to merge styles together and remove the 'key'
and 'parent' properties from the resulting object.

Also fix an issue where the toolbar could shrink below 40px
in height for items with many details fields.