Add value type for a snippet (#81)

Add a new value type struct for a snippet.
New class SnippetRepository (#81)

Add the new class SnippetRepository for the management of snippets.
New class SnippetCompleter (#81)

Add the new class SnippetCompleter. This class provides autocompletion in
the markdown editor for the snippets contained in the SnippetRepository.
Add autocompletion support to markdown editor (#81)

Add autocompletion support for snippets to the MarkdownEditor class using
SnippetCompleter and SnippetRepository.
Add first version of built-in snippets (#81)

Add a new JSON file with a first cut of the built-in snippets.
Add built-in flag to Snippet type
Move management of SnippetRepository

Move the management of the SnippetRepository instance from MarkdownEditor
to MainWindow.
Add default constructor and setContent()

Add default constructor to initialize values and a new method to set the
snippet content and find/remove the cursor position marker.
Several changes to SnippetRepository

* emit dataChanged() signal
* return index of added snippet in addSnippet()
* add method to set content of a snippet
* add method that returns the snippet count
Snippet system configuration (#81)

Add new page to the options dialog for the maintenance (add, edit, remove)
of user-defined snippets.
Snippet to Json conversion

Add static method to Snippet to convert it to a QJsonObject.
Load/Save user-defined snippets (#81)

Add support for loading & saving user-defined snippets in a JSON file.
Merge branch 'develop' into feature/autocompletion-#81

Conflicts:
	3rdparty/hunspell
	app/app.pro
Merge branch 'feature/autocompletion-#81' into develop