Added new panel: CategoriesPanel.
Added the category panel to the main window.
Added new class: CategoriesTreeItemsCreator.
Refactored the CategoriesTreeItemsCreator class.
Changed the type of the categoriesTree from QTreeView to QTreeWidget.
Using the CategoriesTreeItemsCreator in the CategoriesPanel.
Merging with the branch categoryPanel. Fixes #16.