QSelect: Differentiate between mutiple selection and singular via class (quasarframework#7303)

* QSelect: Differentiate between mutiple selection and singular via class

This allows to differentiate between multiple and singular QSelect, so it possible to add styles for a specific case.

* Update QSelect.js

Co-authored-by: Razvan Stoenescu <razvan.stoenescu@gmail.com>

Author: dmitry

ui/src/components/select/QSelect.js

@@ -171,7 +171,9 @@ export default Vue.extend({
     },
 
     fieldClass () {
-      return `q-select q-field--auto-height q-select--with${this.useInput !== true ? 'out' : ''}-input q-select--with${this.useChips !== true ? 'out' : ''}-chips`
+      return `q-select q-field--auto-height q-select--with${this.useInput !== true ? 'out' : ''}-input` +
+        ` q-select--with${this.useChips !== true ? 'out' : ''}-chips` +
+        ` q-select--${this.multiple === true ? 'multiple' : 'single'}`
     },
 
     computedInputClass () {