Use ecm_add_qml_module() from ECM

This MR intended to address the very same issue as frameworks/kitemmodels!49 (merged)

Currently the QML plugin will have a lib prefix under MinGW, which will results Qt apps couldn't load the QML plugin.

The CMake support provided by ECM can be used to resolve this issue and also takes care of a number of other things that related to QML module. This patch intended to make use of it to address the issue.

Since after this patch, the library is no longer hard-coded as a SHARED lib, so it could also make static build easier.

(ps. I'm not 100% sure if I did it correctly so please also review my patch carefully)
(ps. please also help review frameworks/kitemmodels!49 (merged) if possible)
(ps. plasma-framework seems also need to do the same or similar thing btw, I might work on that if this one can be merged)