Add prefix to custom properties

[MoritzLost]

• Add tobii prefix to all custom properties to avoid collisions.
• Use prefixed custom properties everywhere but fallback to the old names to prevent a BC break.
• Add a changelog with update instructions and a deprecation notice.

[viliusle]

1. If we are creating changelog.md, it means it will have similar / duplicated data on https://github.com/midzer/tobii/releases. Maintaining only https://github.com/midzer/tobii/releases could be easier. I am not against it, just we need to think about it.
2. [deleted]

Update: actually code is correct, it fixes issue and my 2 is not valid anymore.

So PR looks good if we are going to add additional file `changelog.md.

[MoritzLost]

@viliusle Personally, I prefer the CHANGELOG.md file – it's easily discoverable and is included in the repository itself, so it's accessible from the code editor. Also, maintaining the changelog in a file means that pull requests like this one can include update information and deprecation notes. I can't do that for release notes, so it's more work for the one who creates the relase in the end. If the changelog is consistently updated with every commit and PR, creating a release is really quick since you can just copy over the release notes from the changelog. Keep a Changelog has some more info on standardized changelogs.

But I don't mind either way, let me know if you want to remove the changelog.