Add THtml::SetIncludePath() to specify a ':' delimited list of directories...

Add THtml::SetIncludePath() to specify a ':' delimited list of directories that should be removed from header files' paths. E.g. abc/inc/Klass.h will be converted into inc/Klass.h if SetIncludePath("abc") was called.
Fix locating of header files that do not match an include path, see http://root.cern.ch/phpBB2/viewtopic.php?t=8126


git-svn-id: http://root.cern.ch/svn/root/trunk@27728 27541ba8-7e3a-0410-8455-c3a389f83636