About dialog: Enhance changelog parsing, add support for new format

[ferdnyc]

I finally came up with the perfect solution to OpenShot/libopenshot#486 and OpenShot/libopenshot-audio#92 (my proposed new Git changelog formats), which have been in WIP status pending compatibility with the About dialog's changelog listing.

This PR updates about.py to:

• Rework the existing parsing into reusable function parse_changelog(), called once per project
• Add a second, alternative parse_new_changelog() to read the new-format files
• Check for both file types, the old-style files in settings/project.log, the new files in resources/project.log.

If it finds a new-style file, it'll read it. If it can't, or it's not found, it'll try for an old-style file. If neither can be found/read, it'll remove that individual tab from the dialog, so that there aren't any empty listings. (If none of the 6 files is present, the Changelog button is removed from the About dialog completely, same as always.)

That means that this can (and should) be merged before any format changes. It'll keep on supporting the old format, and then at whatever point we choose we can switch over to generating new format files and placing them in resources/ instead.

[jonoomph]

@ferdnyc What about the for encoding_name in ('utf_8', 'utf_16'): support for Windows changelog files. Is this no longer needed? I found it necessary due to Windows' git changelog was (for some reason) in the wrong format.

[jonoomph]

I resolved the conflicts on this one, so we can merge it. If we need to handle the different UTF formats for Windows, we can always make a new PR. I'll test this one shortly.

[ferdnyc]

@jonoomph Sorry, missed that question until now.

The different encoding attempts are still there. I moved that code into a reusable function parse_changelog (at the start of about.py), so it can be called for each of the files. But the code there is pretty much the same as before, just DRYer.