Save dialog UI fixes

This brings a few fixes for the save dialog screen

• Changed the icon from "warning" to "dialog-warning"
• Fixed a translation error (added an "a") in the pop-up widget
• Made sure the pop-up widget scales properly

Final result:

Test Plan

• When you open a PNG or JPG file, create a new layer
• Save the file
• The save dialog should pop up

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.