KisPaletteEditor: set storage location on palette change

Document-embedded palettes would save to the document on the first session, but then not save any additional changes to the palette on subsequent sessions, unless the Edit the current palette dialog was first opened and closed before making changes to the palette.

Test Plan

• Open Krita and create a new image.
• Open the palette docker if it's not already visible with Settings -> Dockers -> Palette
• In the palette docker, click Load a palette -> Add a new palette
• In the new palette dialog, click the checkbox Save Palette in the Current Document before clicking OK.
• Add a color to the new palette.
• Save the document, restart Krita, and reopen the document.
• In the palette docker, open the document's palette.
  • The color we added should be there, this is normal and already worked as expected.
• Add a second color to the palette.
• Save the document, restart Krita, and reopen the document.
• In the palette docker, open the document's palette.
  • Both colors should be in the palette, this would not have worked before unless we had first opened and closed the Edit the current palette dialog before adding the second color.

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.
• Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

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.