Regenerate containers

[snipercup]

Fixes #530

Adds a mechanic where containers are reset after some time.

• Adds a spinbox to the furniture editor that allows the user to specify a number of days before regen happens
• When the player is close to the furniture, the calculations and regeneration happens. This means that if you put 0.1 days as a regeneration period and approach the furniture after 10 days, it will only have regenerated once.
• Existing items in the furniture container are lost upon regeneration
• Trees can be looted and will regenerate after 10 days

Also fixes bugs:

• The furniture container sprite wouldn't properly show if it was loaded from a save while empty
• Getting a random mob id was missing from RMobgroup
• Spriteid was missing from RItemgroup

The container sprite on the furniture may look a little wonky at the moment since all the trees show the container sprite. Might add an option to hide the container sprite for furniture.