Add Quest Reward XP slider

similar to https://github.com/azerothcore/Keira3/issues/508

Improve the reward XP result based on level.

[Example](https://classic.wowhead.com/quest=123/the-collector):

---
Want to back this issue? **[Post a bounty on it!](https://app.bountysource.com/issues/91346406-add-quest-reward-xp-slider?utm_campaign=plugin&utm_content=tracker%2F139701226&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://app.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F139701226&utm_medium=issues&utm_source=github).

Start by reviewing the linked Keira3 issue #508 and the WoWHead example for the intended reward XP behavior. Then locate the existing quest reward editor and determine how level-based XP is represented; the work is done when the editor provides the requested XP slider and produces the expected level-dependent result.

Written by the indexing model from the issue text.