Creating claim (2)

# User
1. Equips a bone
2. Clicks on the block of the first corner - which is already claimed.
3. Clicks on the block of the second corner
4. Puts bone away
# Plugin
1. Show remaining claim blocks
2. Show outline of existing claim and error message. Ask the player to click there again to make a subclaim
3. Show selection cross for the corner
4. Remove visualisation
