[VM] Virtual folders of threads

[Uday S Reddy]

In the trunk of VM (revision 1273) I have added a new command `V T'
(`vm-create-virtual-folder-of-threads') for creating virtual folders of
threads instead of individual messages.  For example, to create a virtual
folder of all threads that have messages written by you, you can use
     
    V T outgoing

I found it useful while cleaning up folders for mailing lists.  If you think
of other applications, please let us know.

This will be part of the next release, VM 8.2.0, whenever that happens.  I
have gotten quite busy at work.  So, the VM work is moving very slowly if at
all.

Cheers,
Uday