Add Krita plugin

[amyspark]

Hi @c-koi and @dtschump,

This PR is to upstream the changes I made to plugin-ify GMic-Qt in Krita. This work was done in !581.

It's mostly a port of the existing Krita host, but with the following changes:

• it resides on the same memory space as the host app, so it directly shares pointers to a structure that mimics the old messages
• adds KDE's ECM as a dependency to install the plugin to Krita's directory hierarchy
• links against an exported module of Krita master named "kis_qmic_interface", which is a 1-1 implementation of the old messages as exported functions
• implements all previously unsupported input modes, as they've been made available on Krita master

[c-koi]

@amyspark @cgilles

We would like to thank you again for your contributions to make the G'MIC-Qt plugin work on Digikam and Krita. Having G'MIC-Qt working on different host applications is very valuable to us and we really appreciate the work you've accomplished for this purpose. The improvements of the new plugin version for Krita look good.

Recently, we have thought long and hard about how we can incorporate contributions in the upstream repository gmic-qt. We have written a detailed "Contributing - New host HOWTO" guidelines, and from now on, we will only accept PRs that comply with these rules.

For you, this means we will not incorporate your code or PR (in their current forms) in the gmic-qt repository. We have indeed noticed that this part of the code (for DigiKam) and this one (for the new Krita plugin) are duplicates/variations of the launchPlugin() functions which could break the compatibility with future versions of the plugin, not to mention the few drawbacks which are listed in the guidelines.

In any case, we will be happy to share links to your alternative implementations when we become aware of their locations, here and on the official G'MIC website.

Thanks again for all your efforts.

Sébastien and David.

[hallarempt]

It might be better to completely remove the krita host then because it will not work with Krita going forward.

[dtschump]

Shouldn't we keep it alive for older versions of Krita ?

[hallarempt]

Not in the repository, the last release is good enough for that.

[cgilles]

Please remove digiKam plugin too...

[dtschump]

@boudewijnrempt @amyspark : We have decided to keep the 'old' version of the plug-in for Krita alive, until you release an official build of Krita that contains your new embedded version of the G'MIC-Qt plug-in.
This way, people using older versions of Krita will be able to update to newer versions of G'MIC.
We'll remove it when the new Krita + G'MIC-Qt included will be released.

[hallarempt]

We have long since released a version of krita with our fork of the gmic-qt plugin.

[c-koi]

@hallarempt @amyspark Krita specific code has been removed (65a0e94)