fix self.is_dirty -> page.is_dirty typo

pylint discovered that we were setting self.is_dirty outside of init and indeed that was a real mistake in the code (introduced by me). It was supposed to set page.is_dirty (which is a thing) instead of self.is_dirty (which does not exist for Pathbar()) This is more than a pylint beautification, it is a real bug discovered with pylint's help.

P.S. The impact of the bug is minor to nonexistant, in the worst case we discard and rebuild a page too often. But likely not even that.