fix(dropdown): make "set text" work for custom actions

[lubber-de]

Description

The set text behavior was not working for custom actions whereas such function internally calls the "set text" method.
This PR also fixes showing all items before hiding when a non-multiple dropdown allowed additions

Testcase

Broken

• open the dropdown and enter some non existing value, so the "Add..." appears
• select that new value or press enter
• internal call of set text does nothing (field is empty)
• the whole dropdown menu appears for a blink of a second then hides
  https://jsfiddle.net/mikelogit/874caudh/7/

Fixed

• all fine (selected value is set and menu does not reappear on closing)
  https://jsfiddle.net/lubber/6u8etwf3/1/

Closes

#2955