Fix files with spaces in converter cache;

Fix files with spaces in converter cache;
Delete cached conversions if the converter changes.

	* src/frontends/qt4/QPrefs.cpp
	(PrefConverters::update_converter): delete cached files since the
	new converter might create different ones
	(PrefConverters::remove_converter): ditto

	* src/support/convert.cpp
	(unsigned long convert<unsigned long>): New template specialization

	* src/ConverterCache.h
	(ConverterCache::remove_all): New method, removes all cached
	conversions of a particular converter

	* src/ConverterCache.cpp
	(CacheType): Also store source file format
	(ConverterCache::Impl::readIndex): Handle files with spaces correctly
	(ConverterCache::Impl::writeIndex): ditto
	(various): adapt to changed CacheType

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18378 a592a061-630c-0410-9148-cb99ea01b6c8