Add a new Python plugin called Mono Filter v0.0.1

Add a new Python plugin called Mono Filter v0.0.1

In Photoshop, we can easily change an image into monochromatic bitmap with diffusion dither effect. While this can't be done in Krita, so I made it. This is the very first version of this plugin so it only contains 1 filter, which is "dither effect". After the filter is applied, it will create a copy of the original paint layer and modify it.

Test Plan

1. Open a new document.
2. Make sure there is an active paint layer.
3. Select Filter > Diffusion Dither to see the effect.

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.