Adding movement

Major update for v0.5

• - movable cursor
• - selectable tiles
• - clear screen function
• - X and Y boundaries
• - Can place down active "Current" tile  

Next thing to tackle.

• - Due to the limited functionality of BASIC for the ZX81 not having sprite, i need a way to capture the tile/character data that is at the location where the cursor was moved to and placed back after moved to another location is needed to preserve the overall image as the user moved inside. Perhaps CHRTO (character going to) and CHRFR (character coming from) for placing the CHRFR back its original location.