Allow multiple root goals

[ahitrin]

Currently, only one root goal is allowed. It's a legacy of my previous experiments (e.g., Clokado) which were aimed to solve a single big goal a time. SiebenApp allow to work with more complex goals, and there is no actual need to stick all of them to the single root goal.

Let's try to allow addition of new root goals. This could simplify overall goaltree height and reduce its complexity.

[ahitrin]

Since goal rendering process is being seriously affected by #255, it doesn't seem a good idea to shake this process from the other side. It's better to postpone this feature (again!).

[ahitrin]

Not considered as blocked anymore. A recent commit may draw virtual root goal independently from real goals hierarchy. This is one goal towards current feature. Also, rendering engine that determines positions of goal widgets and edges has been quite well decoupled from any UI implementation in #255.

Most probably, it could be taken into work in one of next releases.

[ahitrin]

I've found there's a little motivation for me to work on this issue. Yes, it could help in reducing overall complexity on large goal trees. But it still doesn't seem significant.

Instead, I'd like to focus on other improvements, like rendering or small (but still important) usability fixes.