Change branching strategy

[Benjamin Höglinger-Stelzer]

This is one of the few projects that sadly do not yet cohere to Git best practices, so the master branch is messy and not suited to guarantee producing working builds. After getting the v3 features done, change branching strategy to:

• master - always builds without errors and can produce a working copy, however might miss latest features
• develop - main "unstable" branch where all the work is done in

Also start using branches and PRs more, this has the nice side effect of GitHub being able to auto-generate proper changelogs.