Some small IDE fixes

* Drag-and-dropping text now moves the text instead of copying it. The
'drag-and-drop to link to new passage' functionality, however, remains
unchanged.
* Undoing the 'drag-and-drop to link to new passage' now undoes both
pairs of inserted brackets, and not just one.
* Opening a passage whose editor window is open but minimised with now
un-minimise the passage.
* Test Build will no longer fail if the story's save destination is set
but doesn't exist (by, e.g. transporting the .tws to another computer).