The page manager now updates the bounding box of every element.
Added a progress bar.
The progress bar now updates its size when the percentage is changed.
Added a default value of 0 for the percent in the progress bar.
The label now recalculates the size of the parent element.

This is needed when the string or the style of a label changes to
realign the oter widgets in the parent container.
Made the setString() function of the line edit work.

There was a bug with resizing the line edit too, fixed it.
Added vertical padding to the horizontal splitter.
Added notification sources (empty, without implementation).