add Ellesmere skin support for spellbook and compact frames

[takiguru]

Adds optional EllesmereUI skin support to What's Training, including the spellbook view and standalone window.
The skin follows EllesmereUI’s settings, with matching controls, fonts, and tab styling. The default appearance stays unchanged when the skin is disabled.
This also requires a small EllesmereUI API addition (SetTabSelection) to keep tab highlighting in sync. The addon still works without it, but the styling and tab highlighting won’t look quite right.

[takiguru]

I also created a PR for Ellesmere to support the tab selection API: EllesmereGaming/EllesmereUI#2273

[fusionpit]

Thanks for this, I will take a look at it sooner rather than later. One thing is that the latest build of the game introduced more avenues for taint when modifying the spellbook in any way. I'm still looking into what this means for this addon, but I may need to "fake" the tab in the spellbook to avoid touching anything Blizzard related.

[takiguru]

Thanks for taking a look! No pressure at all, especially with the new taint issues. If you end up changing how the tab works, just let me know if there’s anything I should adjust in the PR. Happy to adapt it to whatever approach works best.