Improve CI

[nyalldawson]

Given the heavy improvements currently being done, it makes sense to improve the CI first 🥳

This PR:

• replaces the travis build with a github workflow based build
• adds a macos build
• adds a clang static analyser build (and fixes some issues it flags)
• adds cppcheck workflows (note that this is run on two different environments as gdal/proj/qgis maintainers found that different warnings are raised with these two different versions!)

https://github.com/qgis/libdxfrw also has a workflow for periodic builds using coverity scan, but that workflow isn't so easy to port (requires a repo maintainer to add the private token for the build submission, and will first require coverity setup for this repo. I can do that if desired, but I'll still need someone to add the private token to this repo)

[lordofbikes]

@nyalldawson please, don't be cross with me, but I have to put a break on your efforts for the moment.
I haven't looked at your pull requests #43 and following yet.
But my worry is, that it will become just more pain to merge them than the last series.
I communicated in #28 already, that I merged all pull requests till #42 and I merged the mergeLibreCAD2 branch too.
Regardless you pushed to some already merged branches. But no problem, I think I can cherry-pick them.

For the new pull request, following this one, I'll have a look later, when I have finished my changes.
So I recommend to wait now for my push, that you can continue from a more up to date base.

I understand your intention of keeping things separate in branches. But with this amount of changes, touching the same file in multiple branches, makes merging a pain, lots of conflicts. So we possibly find a better way to handle this with less efforts. I have some ideas, but no time yet, must run.

[nyalldawson]

@lordofbikes

No problem, I totally understand! I'll hold off on the prs until you're ready

[lordofbikes]

@nyalldawson I finally found the time to push my local merging and conflict solving.
I had to update instructions in LibreCAD_3 for the new LibreCAD_3 branch, because the new master is now incompatible with the LibreCAD_3 interface definition. See description of #20 for details, it will affect QGIS too.
I also updated README.md to sign that this is the successor of the former SourceForge project.

Before I can come back to any new pull requests I have to handle some urgent vulnerability reports before Oct. 31.
So I have to ask for a bit more patience concerning your further pull requests.
You should also not pull this new HEAD to your pull request branches, you can leave them untouched by now.

Many thanks for your efforts!