Implement option to modify fixed addresses with base address change

At the moment fixedAddresses are always fixed, so if a base address is changed then fixed addresses stay in place.

eg. base address = 0x0, with a fixed address of 0x15 implies distance from base address to fixed address is 0x15; changing base address to 0x10 with fixed address staying at 0x15 implies the distance from base address to fixed address has changed to 0x5. This may, or may not be a good thing depending on the circumstances.

Need to implement an option to maintain the offset between base address and fixed addresses when base address is changed.

eg base 0x0, fixed 0x15 => offset 0x15; with "maintain offset" option base changed to 0x10 => fixed address becomes 0x30 to maintain 0x15 offset.