Add system to calculate amount of xp when missed

[BitTim]

Create a button in the create history entry popup, next to the xp amount, in case you skip the xp screen in game. This will prompt the user to input their current BP level and xp within that level, calculate the difference between the previous known state and fill this difference into the xp amount field.

• Add button next to xp amount in EditableHistoryEntryPopup
• Open either another popup stacked over active one, or make input fields appear for level and xp within that level
• Calculate the XP difference
• Fill calculated value into the xp amount field in EditableHistoryEntryPopup