424514 Plugin-ification of GMic

This MR converts GMic, currently a subprocess with its own executable, into a self-hosted Krita plugin. The objective of this work is to enable GMic support in macOS, while also getting rid of the whole separate-executable etc. This also fixes issue #44 in the gmic-qt repo (not being able to run it on CentOS).

The changes involved are as follows:

• Added a small shim library, currently named kisimageinterface, that serves as the middleperson between Krita, GMic-Qt and kritaqmic (the KPlugin that loads the GMic KPlugin in turn).
• Replacement of the current QSharedMemory-based infrastructure with a new struct, KisQmicImage, that hosts the data that's shipped between kisimageinterface and GMic-Qt. It's an almost-direct replacement of the existing messages.

These changes have been integrated in a new GMic-Qt host, krita-plugin, whose code is attached as a series of patches to the existing GMic-Qt download.

This MR implements support for Linux AppImage, Windows, and macOS. In all cases, an extra step has been added to the Krita build target, which runs the external projects.

Please note that I have removed Android support from this MR. See bug 428995.

BUG: 424514

BUG: 431151

BUG: 429851

Test Plan

Build your own AppImage by running build-deps and then build-krita. Then, stop! and run build-plugins with the same parameters as build-deps. You can then pack the image as usual.

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.