Extend the editor with Drag&Drop support for resources

[gurkenlabs]

Currently, we need to use the Import dialogs in order to import any resources. We need to make the Resources List aware of Drag & Drop.

Also, the shortcut CTRL+V in case any resources are in the clipboard and the resources tree is selected should be supported

The reverse operation should also be supported: Dragging/Copying CTRL + C Spritesheets to the filesystem should create images (or other resource files -> similar to the export functionality)