Fix bug 2868

Fix bug 2868
	* src/insets/insetgraphics.C
	(findTargetFormat): return pdf for vector graphics

	* src/format.h
	(Flags): new enum describing format flags
	(flags_): new member variable for format flags
	(vectorFormat): new, tell whether a format can contain vector graphics

	* src/format.C: adjust to flags changes

	* src/frontends/qt[34]/QPrefsDialog.C
	(QPrefsDialog::QPrefsDialog): handle new vector flag
	(QPrefsDialog::switch_format): ditto
	(QPrefsDialog::updateFormatsButtons): ditto
	(QPrefsDialog::new_format): ditto
	(QPrefsDialog::modify_format): ditto

	* src/frontends/qt3/ui/QPrefFileformatsModule.ui
	* src/frontends/qt4/ui/QPrefFileformatsUi.ui
	(vectorCB): new checkbox for vector flag

	* src/lyxrc.C
	(LyXRC::read): read vector flag
	(LyXRC::write): write vector flag

	* lib/doc/Customization.lyx: document format flags

	* lib/configure.py
	(checkFormatEntries): Add vector flag to some formats

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15205 a592a061-630c-0410-9148-cb99ea01b6c8