Rebase 5.12 branch on xenial
(yes, rebasing in a pull request is stupid but there's no nicer way to do this in github. Merging would mean force pushing the 5.12 branch with the content of this PR or similar)
Based on #38 . Tested on Arch Linux with Qt 5.14 using qqc2-gallery. These commits (except "Replace QQuickColorImageProvider with QQuickColorImage") probably also work fine on Qt 5.9 so picking those into the xenial branch would also be a good outcome of this :)