Fix Tab protocol methods being unduly invoked
Fix Tab protocol methods being unduly invoked

While switching between Unified layout and Vertical or Horizontal layout,
I saw various exceptions in log:
-[ArticleListView tabUrl]: unrecognized selector sent to instance
-[ArticleListView isLoading]: unrecognized selector sent to instance
-[ArticleListView pageDown]: unrecognized selector sent to instance

josh64x2/vienna-rss,barijaona/vienna-rss,barijaona/vienna-rss,barijaona/vienna-rss,barijaona/vienna-rss,ViennaRSS/vienna-rss,josh64x2/vienna-rss,ViennaRSS/vienna-rss,ViennaRSS/vienna-rss,ViennaRSS/vienna-rss,josh64x2/vienna-rss,josh64x2/vienna-rss,josh64x2/vienna-rss,barijaona/vienna-rss,ViennaRSS/vienna-rss