[SPELLCHECK] hunspell library and first version

Add hunspell library as git submodule and a very first
implementation to the MarkdownHighlighter class
[SPELLCHECK] better integration of the spell check

Highlighting of syntax and spelling errors works now.
Todo:

* create separate spell checker class
* add action to find spelling errors and offer corrections
* add option to turn spell check on/off
* add possibility to select the dictionary
[SPELLCHECK] Add new class SpellChecker

Move spell check functionality to a separate class hunspell:SpellChecker.
Update travis build file
Try to fix compilation for travis
[SPELLCHECK] suggestions in context menu

Add suggested replacement for mispelled words to the custom context menu
of the markdown editor.

The mispelled word will be replaced by the suggestion of the spell checker
when the user triggers the action.
[SPELLCHECK] enable/disable check and choose language

Add options to enable/disable the spelling check and choose
the language of the dictionary.
Merge remote-tracking branch 'origin/develop' into feature/spellchecking-#6
Merge branch 'feature/spellchecking-#6' into develop