FindAdv: Special handling for \dot{i} and 'ß'

FindAdv: Special handling for \dot{i} and 'ß'

Different behaviour in regexp{..} for 'İ' and 'ß':
1.) lowercase routine for 'İ' gives 'İ', so that if we are searching
  while ignoring case, the string '\dot{I}' is converted to '\dot{i}'.
  In this case we have to change it to 'İ' (instead of 'i', as one would expect).

2.) If 'ß' is inserted via keybord on fresh created regexp box it appears as \lyxmathsym{ß},
  if pasted from the lyx-screen it appears as \text{ß}