source code management refactor

Hi,

I've recently pushed a small patch to add minimal Fossil support to
Kate's Project plugin.  It felt so good to contribute to my favourite
KDE application after many (many!) years of using it!

The next step, of course, is to get more Fossil functionality; ideally
getting it to parity with the current Git support.

Looking at the code, I found several places like "if git, do this
very-git thing", which of course, doesn't scale to multiple SCM
backends.  I'm not a fan of gratuitous inheritance, but it seems the
best way forward.  As a nice bonus, it should make it easier to add
similar features to Mercurial, and maybe Subversion.

Now, I haven't done any real C++ since Qt3 days, so I'd like to ask
for some comments before going too far into a bad design.

I've just pushed a new branch:
https://invent.kde.org/javier/kate/-/tree/work/javier/fossil_branches
It adds a `scm/` directory with a base class and a class for each SCM
supported.  So far the features:

- SCM detection (all SCMs)
- get managed files (all SCMs)
- get SCM icon (for Git and Fossil)
- get current branch name (Git and Fossil)
- list of branches (Git and Fossil)
- checkout branch (Git and Fossil)
- new branch (Git and Fossil)

have been moved to use these new classes.

Is this list the appropriate place to discuss design?  or would it be
better to open a PR and do it there (even if the code is still very
much in process)

thanks!

-- 
Javier