Introduce usage of KF5::Sonnet to perform basis spell-checking in caption text fields.

We do not use volumntary the more complex KF5::KTextEdit as it introduces extra keyboard shortcuts which break the user workflow
while tagging items in collection or navogating in albums.
The spell-checking language is set with the text language code from caption settings.
The dictionnaries must be installed of course from one of supportted backend as ASpell or Hunspell