* Add a DEFAULT state to InsetGraphicsParams::DisplayType.

* Add a DEFAULT state to InsetGraphicsParams::DisplayType.
* InsetGraphics::updateInset will interogate lyxrc.display_graphics if
params.display == DEFAULT.
* Ensure that the "default" display is read to and written from file correctly.

Next to do: pass a GraphicsParams struct to the GraphicsCache when adding
an image rather tahn just the filename and use this to decide exactly
how to display the image. Will therefore be able to strip out the
lyxrc stuff from ImageLoader et al.

Angus

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@3476 a592a061-630c-0410-9148-cb99ea01b6c8