[kcms/users] (UX) Add a cancel button when creating a new user

Rémi Piaurequested to merge
redmie/plasma-workspace:BUG450420 into master

Add a cancel button when creating a new user in kcms/users

  1. Disable and hide "Add User" button when displaying CreateUser interface
  2. Only display "Cancel" button in desktop mode, mobile can use back button on toolbar

Also added icons as there are now 2 buttons.

Reason for the change

Improve user experience in systemsettings>users

Test plan

Tested with back and forth between the user list and the create user interface using kcmshell6 with and without QT_QUICK_CONTROLS_MOBILE=1 flag (mobile).

Screenshots or screen recordings

Desktop Mode (new cancel button)
Before After
Screenshot_20250707_112512 Screenshot_20250707_112427
Mobile mode (no changes)
Before After
Screenshot_20250707_112920 Screenshot_20250707_112837

Bugs fixed

BUG: 450420

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