As a follow-on to moving the jfx repo to git, we need to decide on our branching model.

Here are some high-level thoughts:
- we probably want to use branches in main repo for stabilization of feature releases
- it might or might not be a good idea to have branches in the main repo for update releases (note that it is possible to have different maintainers / policies / etc)
- in any case, we need to adopt a consistent naming policy for branches: to match jdk, no "-" between jfx and NN, e.g., jfx13 not jfx-13