Add LColor::graphicsbg and use it in the graphics and figure insets.

Add LColor::graphicsbg and use it in the graphics and figure insets.
Ensures that a figure with a transparent colour can be viewed
easily if the LyX screen's background colour is dark.

Prevent deletion of the image file if no conversion is required.

This patch works like a charm with the (old) figinset. When testing
the graphics inset, however, I find that colour "None" in an XPM file
is displayed as black. I have no idea why. This appears to be true of
some other graphics viewers also. E.g., KDE 2's pixie displays "None"
as black but the venerable xv works fine.

Maybe it's my version of the xpm library, but v3.4.11 appears to be
right up to date.

Can people test on their machines and/or offer suggestions here?
Angus

p.s. You can test with the XPM files in lib/images. They will no longer
be deleted if you load them into LyX!

A.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@2777 a592a061-630c-0410-9148-cb99ea01b6c8