Reset palette docker's model on column count change

This will refresh the palette docker's view so we can see the change right away.

Normally, if a palette is edited such that only the number of columns is changed, the change won't show up immediately. It will require either changing the row count as well in the same edit, or switching back and forth between different palettes.

Test Plan

• Open a krita document and select a color palette from the palette docker
• Open the palette editor and change only the number of columns
• Click OK and observe the number of columns appearing in the palette view

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.

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.