Bugfix: New image dialog doesn't allow set resolution in pixels per centimeter

I added the option to set the resolution when creating a new file.

Test Plan

1. File > New...
2. Set a resolution in Pixels/Centimeter (the new option!) and create the file
3. Image > Properties

Even though the dialog shows the resolution as ppi, you may notice that it is in fact the conversion of the ppcm resolution set previously.

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.