Allow client-side channel buffer merges, and associated changes.

networkmodel.cpp: Mark ChannelBufferItems as being a drop target, to
allow for channel buffer merging.

bufferviewfilter.cpp: Rework BufferViewFilter::flags() to make it more
clear what's going on, and remove the restriction that made
QueryBuffers the only merge-able buffers.

bufferview.cpp: Rework BufferView::dropEvent() to add a bunch of
comments, and allow ChannelBuffers to be merged as well as QueryBuffers.
ChannelBuffers can only be the source for a merge if they are not
currently joined, to prevent UI weirdness of being in a channel but not
having any associated UI elements.

Resolves GH-220.

(cherry picked from commit 6ee26fd6d0a163314002616d277e5444f11b7720)