Add useful commands

What have you done in this MR?

• Implemented addxp command. Add progress/xp to a skill with more granularity, not just levels.
• Implemented charge command. Get and set the (internal or distinct) charge of an item.
• Implemented setslayertask command. Set the slayer task monster/npc, with optional task amount.
• Some refactoring and bug fix.

Are there any tricky things testers should keep an eye out for?

• No

• I have tested these changes thoroughly.

• I acknowledge that this contribution will be released under the AGPL license.