BaseTool: add member Editor*
minor
SelectTool: use cross cursor for both bitmap/vector layer
ScribbleArea: extract setCursor() to one place
ScribbleArea: move method pencilOn() to toolSet

--HG--
branch : ToolOn
ScribbleArea: move method eraserOn() to ToolSet

--HG--
branch : ToolOn
ScribbleArea: move method selectOn() to ToolSet

--HG--
branch : ToolOn