[nemo-qml-plugin-utilities] Add Screenshots component

This patch provides the ability to trigger screenshots from QML and save them to a specified directory. Being able to do this from QML is useful for test scripts.

There are two modes of use: by default it takes screenshots from the desktop root window, so that composed elements such as the virtual keyboard show up.

If you set the "target" property, then it takes screenshots of the target qml item by asking the graphics scene to render it to a pixmap. This renders the item as seen in the scene, so with transformations applied and with other items possibly obscuring it.