Add python plugin Channels to Layers

As proposed (https://krita-artists.org/t/channels-to-layers/5350/5?u=grum999), add plugin to default Krita plugins

Test Plan

Launch Krita Install plugin Execute plugin

Tested on Krita 4.2.9

Formalities Checklist

• [n/a] 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.