Deal with a UI bug discussed on the list.

Deal with a UI bug discussed on the list.
The effect is this: If you try to open a file that is already open,
LyX (i) checks if the buffer is dirty, and just switches buffers if
it is not; (ii) if it is, you get this message:
 The document %1$s is already loaded and has unsaved changes.
 Do you want to abandon your changes and reload the document?
    [Reload] [Keep Changes]
If you hit the latter, you just get switched to the document.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@19331 a592a061-630c-0410-9148-cb99ea01b6c8