fix circular dependencies between qt interface and "vout window" modules

this merge request tries to solve the circular dependency between Qt as an interface module and Qt as a "vout window" module. the issue is that the interface module might be released before the vout window, destroying resources that are still in use by the vout window.

In order to achieve this:

the first two commits are mostly "sed" commits to prepare what follows, the actual changes in those are in qt.cpp/hpp