[qt-components] Expose a gconf key for framebuffer orientation in MDeclarativeScreen

This gconf key is also used by Sailfish Silica. Currently it's exposed
directly through a new property of MDeclarativeScreen, but it should
be used for more logic in later work.
Merge pull request #7 from special/qtquick2

[qt-components] Expose a gconf key for framebuffer orientation in MDeclativeScreen