Embedding: After another round of EmbeddedFiles changes, embedded figures should...

Embedding: After another round of EmbeddedFiles changes, embedded figures should more or less work now.
	* src/insets/InsetExternal.cpp: fix compiling. (Does not work yet)
	* src/insets/InsetGraphics.cpp: use latexFilename to produce output.
	* src/insets/InsetGraphicsParams.cpp: use availableFile to show preview.
	* src/EmbeddedFiles.h|cpp: More things are moved from EmbeddedFiles to EmbeddedFile
	* src/frontends/qt4/GuiGraphics.cpp: remove embeddable-related operations.
	* src/support/FileName.h: make isReadableFile virtual

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@22410 a592a061-630c-0410-9148-cb99ea01b6c8