Add functionality to group skills within a plan

[doomlord289]

For each of my characters, I have a long-term skill plan that has all the skills I want to train and the order I want to train them in. On some of my specialized characters, it would be nice to group skills together as sort of a sub-plan that has a priority in the main plan, but each skill within the group can be prioritized independent of the main plan.

For example, on my dread alt, I have a grouping of skills to train to fly the Phoenix, then the Zirnitra. Within my main plan, I'd like to have an entry that I'd name "Phoenix Basic" to hold the skills required to fly the fit and ship, then another entry "Phoenix Max" to that has the skills to max out the ship and fit, then similar groupings for the Zirnitra. If I wanted, I could order my main plan as "Phoenix Basic", "Zirnitra Basic", "Zirnitra Max", "Phoenix Max", but after training "Phoenix Basic", decide to re-prioritize and train the "Phoenix Max" group instead.

Today, I just use the priorities of the skills and sort them that way, but if I decide to re-prioritize, I need to lower (update from, say, 20 to 40) the priority on the Zirn skills, then increase (update from, say, 30 to 20) the priority on the Phoenix skills, then increase (update from, say 40 to 30) the priority on the Zirn skills because on a longer plan, 99 priorities sometimes gets a bit tight if leaving a few buffer priorities to handle re-prioritization like the example.