More modernization and cleanups

[nyalldawson]

No description provided.

[lordofbikes]

@feragon before merging this and succeeding PR's let me check about #20.
I hold #20 back because of changes in the interface, which should be implemented in @LibreCAD/librecad-3 parallel, to avoid conflicts when new users download both to build.
I haven't checked yet, but some issues fixed by @nyalldawson, may already be fixed in my branch.
Alternatively, we can merge these new PR's and mine into a new branch or create a tag for the latest commit to use with LibreCAD_3. Or maybe both.

[nyalldawson]

@lordofbikes

Alternatively, we can merge these new PR's and mine into a new branch or create a tag for the latest commit to use with LibreCAD_3

I'd suggest the second option - have a separate branch for librecad3 and keep the master branch as the recommended upstream for cross-application use

[nyalldawson]

For reference I've started collating fixes in https://github.com/qgis/libdxfrw . I still plan on migrating them all here when you're ready for contributions, so ping me when you're ready! 👍