Merge pull request #4966 from owncloud/text_preview_blacklist

add blacklist to txt preview backend

* owncloud/text_preview_blacklist:
  rename variable in testIsTransparent
  rename testTxtBlacklist to txtBlacklist
  move fileView object initialization to testIsTransparent
  use dataProvider for txt blacklist test
  add test for txt blacklist
  add test data for cal and contact preview
  add blacklist to txt preview backend