unifying libdiff2

[David Nolden]

I was the one who once forked libdiff2 many years ago (probably around
2007 or so), and it was mainly for comfort reasons, because libdiff2
would have needed a quite big refactoring to be nicely usable by both
Kompare and KDevelop:
* libdiff2 was a static library living in a subdirectory of the at
that time unmaintained kde4 kompare port
* libdiff2 contained a lot of UI code, while we were only interested
in the 'model' code (that's why I commented out quite a bit of stuff)

While it is nice to share some code, I'm also not in favor of adding
huge dependencies like 'whole kdesk'.

Having the code shared can also make life significantly harder
regarding changes in the library: How should the correct versions of
the library and the application be synced together? We experience this
again and again through the split between kate and kdevelop. Therefore
we should ask the question whether KDevelop and Kompare really have so
much in common, that they will actually _profit_ from each others
potential future improvements in a shared libdiff2. The word 'fork' is
often interpreted more evil than it actually is. ;-)

Greetings, David