Add QtCreator 4.15.x

[Bjoe]

This change will update the sourcetrail plugin for QtCreator 4.15.

Since QtCreator is using cmake as build system, this build is also updated to cmake build.
Also it uses github action instead of appveyor and travis.

All this solution are taken from the QtCreator "it self" See https://doc.qt.io/qtcreator-extending/first-plugin.html

This PR is yet only a draft/proposal, because I only test it yet for QtCreator 4.15.
It might be also working for 4.14 but I didn't test it yet because for 4.13 and 4.14 there is another PR #25
But I'm really sure it will not build for 4.12.
I'm not sure yet what the "strategy" is for older plugins. At first I only implemented the simplest solution.
Also MacOS build must be fixed.

[Bjoe]

Unfortunately gihub action are not enabled here. If you will see this in action see here -> https://github.com/Bjoe/qtc-sourcetrail/actions/runs/857620457