Fix bug #6451 (last part): Crash when interacting with buffer-dependent dialog with...

Fix bug #6451 (last part): Crash when interacting with buffer-dependent dialog with no buffer open.

- First, the comment for isBufferDependent is corrected. It seems that the actual use of this function differs from the comment. As the comment said, I decided to close all dialogs that were buffer dependent, but this didn't seem to be correct for the view source pan, the outliner, and find-and-replace.

- Second, the dialogs that are buffer dependent are now closed, but dockviews are not, except for the spellchecker pane, which really depends on an open buffer, but I can't test that.

So, please test whether the spellchecker dockviewed window behaves as one expects.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@34291 a592a061-630c-0410-9148-cb99ea01b6c8