Convert the global OSD and the image OSD to 1080p

The layers are scaled to match the actual screen size. Since most
TVs now use 1080p it seems to be a good choice for the layer size.
This means for 1080p the layers are their actual size. It still
works for other sizes.
expose stage again
add todo
Smaller enhancements for a beta release

The image plugin now supports to set the blend-mode again, the
slideshow is fixed, and rotation is saved in beacon. Besides
that, this commit includes code cleanup, doc updates,
and smaller bugx fixes.

I consider this plugin ready for a Freevo 2.0 beta release