Investigations into using Qt in a plugin

The typical/classical use of Qt is where the entire application is written using Qt. But in some situations Qt is used to implement a component that plugs into another app, for example audio/music plugins, or extensions to 3D authoring tools.

In this situation Qt is operating under some different rules than normal:

• Qt doesn't control the application lifecycle, the host application does
  • The host is the one with a main(), and the one starting and running the event dispatching
• Qt does not control the application's window hierarchy, the host app does
  • The host decides how and where to incorporate UI elements created by the Qt code

There's also a possibility that the host itself, or other plugins, may be written in Qt, which necessitates stricter requirements on avoiding symbol clashes of various sorts, clashing global data, conflicting environment variables, etc.

As the use-case of using Qt in a plugin scenario is and should be supported, we want to get a full overview of the various sharp corners and considerations that come into play for this use-case, and to build up test coverage for the use-case. This epic tracks the work to achieve that.