changed implementation of PanelSelectionHandler so that custom panelsele...

...ctionhandlers can inherit that class if they are built in the same project. Also added a Partial Selectionhandler that selects objects that are partially selected if both mousebuttons are pressed.

PartialPanelSelectionHandler is activated by adding a custom extension in your project like this:

[ExtensionFor(typeof(Panel), OverrideExtension = typeof(PanelSelectionHandler))]
public class CustomPartialPanelSelectionHandler : PartialPanelSelectionHandler
{
}