Provide minimal qt with qmake buildsystem in public stacks
Context
We currently provide qmake as a standalone in our buildsystems public stack, but almost all users will only want qmake to use with Qt. As a result if we're going to provide this we should include the minimal level of modules a Qt user would expect. @rdale_ct recommends that we use the Qt essentials described here
Description
We should include a minimal qt as part of our publicly available build systems. It will be easier to maintain if we just use LTS rather than the latest version.
Qt is pretty big, so if we want to keep our buildsystems stack minimal it may be a good idea to provide it in a separate stack, so people who don't need qt don't have to include it when they junction from us.
Acceptance Criteria
Other users of BuildStream can junction from freedesktop-sdk and gain access to a minimal version of Qt.