Drag and drop move for project explorer

[schleifenkauz]

Files and directory can be copied and pasted since #13, but it would be more intuitive to move project items with a drag and drop gesture.

• The user should be able to select multiple project items and then drop them into another location
• If dropped on a directory the items should be moved inside the directory
• If dropped on a file the items should become siblings of this file
• The items should be moved and not copied, that is they should be removed from the source location
• No items that are part of the same tree path can be dragged simultaneously
• This issue is probably connected to #14