Fix stylelint errors

Problem/Motivation

There are a few stylelint errors that should be fixed:

menu_select-2.0.x/css/menu_select.css
   39:78   ✖  Expected double colon pseudo-element notation                                    selector-pseudo-element-colon-notation
   62:3    ✖  Unexpected vendor-prefixed property "-ms-transform"                              property-no-vendor-prefix
   63:3    ✖  Unexpected vendor-prefixed property "-webkit-transform"                          property-no-vendor-prefix
   67:90   ✖  Expected double colon pseudo-element notation                                    selector-pseudo-element-colon-notation
   78:90   ✖  Expected double colon pseudo-element notation                                    selector-pseudo-element-colon-notation
   88:78   ✖  Expected class selector ".menu-select-menu-link--has-children" to be kebab-case  selector-class-pattern
   88:114  ✖  Expected double colon pseudo-element notation                                    selector-pseudo-element-colon-notation
  101:78   ✖  Expected class selector ".menu-select-menu-link--has-children" to be kebab-case  selector-class-pattern
  105:78   ✖  Expected class selector ".menu-select-menu-link--has-children" to be kebab-case  selector-class-pattern
  105:123  ✖  Expected double colon pseudo-element notation                                    selector-pseudo-element-colon-notation

✖ 10 problems (10 errors, 0 warnings)
  7 errors potentially fixable with the "--fix" option.