Improve Kaidan portability

I decided to merge my current build system changes separately from the QXmpp move to simplify the transition.

This merge request adds support for Windows and macOS builds, as well as a portable AppImage build script.
Small code changes were required, including a switch to Qt.labs.platform's FileDialog and a list of static plugins.
The Kirigami module update was done due to my static build fixes there.
Pipeline breakage is solely due to a build script rename.

Support for qmake builds is now rudimentary and will hopefully be removed in one of the following weeks.