logic: Directory list box now restricts the user to select files, directories or both if required

* split QtDirectoryListBox and QtStringListBox
* renamed QtDirectoryListBox to QtPathListBox
* extracted QtListItemWidget class to own file
* renamed QtListItemWidget to QtListBoxItem
* added SelectionPolicyType to QtPathListBox to allow for specifying if selecting files, directories or both should be allowed.

src/lib_gui/qt/element/QtStringListBoxItem.cpp

@@ -0,0 +1,14 @@
+#include "qt/element/QtStringListBoxItem.h"
+
+#include "qt/element/QtStringListBox.h"
+
+QtStringListBoxItem::QtStringListBoxItem(QtStringListBox* listBox, QListWidgetItem* item, QWidget *parent)
+	: QtListBoxItem(item, parent)
+	, m_listBox(listBox)
+{
+}
+
+QtListBox* QtStringListBoxItem::getListBox()
+{
+	return m_listBox;
+}