Ultiboard: Fix part selection

[iwane]

Make part selection select parts only on active layer. It is very frustrating when Ultiboard selects and drags e.g. a connector on Top layer along with (intended to be deragged) a resistor on Bottom. Ultiboard also selects locked parts on inactive layers, in situations like in the attached screenshot. I can select the locked part (which is SMT part placed on Top) when dragging parts on Bottom - this is VERY frustrating...