make FileName::isZipped more efficient by caching previous results

	* src/insets/insetgraphics.C
	(copyToDirIfNeeded): replace file_in and zipped arguments with a
	FileName argument
	(InsetGraphics::prepareFile): adjust call of copyToDirIfNeeded

	* src/support/filename.C
	(FileName::FileName): set zipped_valid_
	(FileName::set): ditto
	(FileName::erase): ditto
	(isZipped): use zipped_

	* src/support/filename.[Ch]
	(zipped_): new cache for isZipped()
	(zipped_valid_): new, tell whether zipped_ is valid

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14376 a592a061-630c-0410-9148-cb99ea01b6c8