Prevent the skill calculator xp input fields from resetting unintentionally.

[geheur]

Currently when you open the skill calculator the current experience field will be your current xp in that skill, but if you then click the level field and then click anywhere else without changing the level, the current xp will change to the minimum xp required for that level. This also happens if you've inputted a custom xp quantity and accidentally click the level field. It also affects the target xp field.

This is not intuitive, or as far as I can tell, useful, and this PR changes the behavior to not update the xp field unless there was a change in the level field since it came into focus.