Per-X Skill Support

Support Per-Battle/Per-Event skills better.

Sub-tasks:
   ☐ Add 'show uses' to skill usage model
   ☐ add 'uses format' to skill usage mode '/Ev', '/Twi', default blank (maybe)
   ☐ add 'uses' field to skill form / skill model
   ☐ Group skills by usage/summary, add uses together.
   ☐ for skills configured that way, show Nx - [summary] on the sheet