Make automatic layer name suffixes optional

Adds options to turn off 'Merged' and '(pasted)' suffixes being automatically added to layer names.

Thread on Krita Artists

Test Plan

Settings are under General/Miscellaneous.

1. Merge a group layer and confirm the name matches the current setting.
2. Paste from the clipboard into an existing image, and Paste into New Image, confirm the names match the current setting.

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.