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.