DefaultTreeItem constructors refactored

Refactoring done to avoid code duplication and to clean
a code a bit. Also one additional constructor added:

DefaultTreeItem(Tree tree, int cellIndex, String... treeItemPath)
Tests added for DefaultTreeItem(Tree, cellIndex, String...) constructor - also tests' internal createTreeItems() methods extended to support more tree columns
SWTBot references removed from all RedDeer manifests
Log4j references removed from RedDeer (fixes #176)
Deprecated methods removed from WidgetLookup (fixes #229)
AsyncExec method returning value removal because return value is not reliable