Simplify MainWindow UI

use QStackedWidget instead of multiple QSplitter childrens for the web and
HTML source views. This also makes the code for the splitter handling in
MainWindow simpler.
Improve perceived performance (#100)

Calculate the delay of the HTML processing based on the
number of characters in the markdown document instead of
a hardcode value of 100 ms.
Improve calculation of splitFactor

Take the handleWidth into account when calculating the split factor for
the QSplitter of the MainWindow. This prevents expansion of the right
view when the view is collapsed and the main window is resized.
Improve perceived performance (#100)

Do not update the web view when it is not visible.