prevents too many slide switches

…ithub.com/hicknhack-software/CuteMarkEd into hicknhack-software-feature/reveal.js_slide_position_tracking

Moved the logic for the line to slide mapping from the MainWindow into the separate SlideLineMapping class and refactored the code to be more "Qt-like".

Refactoring of #181

Add lineForSlide() method that returns the line number for the passed
slide and use it in MainWindow.

In template_presentation.html we setup the slide separator to be
surrounded by empty lines. The logic in SlideLineMapping is adapted
accordingly and new unit test were added.

Add slideForLine() method to the SlideLineMapping class. It returns the
slide for the passed line number.

A ViewSynchronizer implementation is responsible to keep the Markdown editor and the HTML preview in
sync regarding its position. Currently there is just the RevealViewSynchronizer implementation for this base class.

Refactor the MainWindow class and move all code regarding the view synchronization to the RevealViewSynchronizer class.

Add implementation of ViewSynchronizer class that synchronizes the
Markdown editor with the HTML preview and move the code from MainWindow
class to HtmlViewSynchronizer class.