Added 3 new properties for text displays to use: billboard, background colour, and background alpha.

• They're pretty self explanatory:
  • Billboard - changes the display billboard
  • Background - changes the colour of the background
  • Alpha - changes the transparency of the background
• These new properties are modifiable via command /textdisplay properties <id> <property> <value>
• Might add a GUI option to modify them in the future, but no promises
• These were not tested as extensively as other additions I usually make, so please report any bugs or anything to me on Discord: @cheezburga

Also some general clean up.