Lib::SplittableArea: Add widget in area

Add widget list in area
Create some widgets in the test create function
Paint a color in widget for testing purpose
SplitterWidget: Add widget (in a scroll area)
SplitterArea: Define functions
SplitterArea: Implement verticalSplit() and horizontalSplit()

- Add the first SplitterWidget in SplitterArea constructor
- Move test creation to the main -> remove function createSomeHandlers()