Extend saveBufferIsNeeded with a parameter indicating whether the buffer is going...

Extend saveBufferIsNeeded with a parameter indicating whether the buffer is going into a hidden state. If so, and the user discards his changes, the buffer will be reloaded to discard the changes.

Now, the option "Hide Tab" uses closeBuffer and asks for saving intead of instantly removing the workArea.

This is part of bug #5893: we try to make sure that there are no dirty hidden buffers.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@31043 a592a061-630c-0410-9148-cb99ea01b6c8