Header cleanup / Link interface (was kdewidgets: fix build)

[Stephen Kelly]

Stephen Kelly wrote:

> Qt5::Core is a public dependency of Qt5::Widgets (as is Qt5::Gui), so
> there's no need to try to be exhaustive with dependencies which cmake will
> add anyway. You only need to add 'leaf modules'.

To be more clear. I'm saying only what CMake requires. Public dependencies 
(if correctly listed) can't be removed or changed without breaking binary 
compatibility.

For policy of what to do in KF5, don't ask me :). I prefer minimalism but 
I'll let others figure out what to do there.

Thanks,

Steve.