Improved skills support, secondary skills and socketed gem modifiers

[brather1ng]

First part of #546.

Most of this happened in the Engine (see PoESkillTree/PoESkillTree.Engine#6 and PoESkillTree/PoESkillTree.Engine#7). This does mainly two things:

1. Split skill parsing into two steps: gem parsing and skill parsing.
2. Allow modifying the skills in between: additional level and quality (values come from the Engine) and enabling/disabling skills of gems that have secondary skills individually.

For active skills with secondary skills, enabling/disabling them can be done in the Skills UI: