Get Size count wraps around over 32,768K

[inexorabletash]

Describe the bug When enumerating multiple volumes, the Get Size "Space used on disk" counter wraps when it goes over 32MB.

To Reproduce Steps to reproduce the behavior:

1. Start DeskTop
2. Select two volume icons where the total size is > 32K
3. Special > Get Size

Expected behavior Accurate total count

Screenshots

Root Cause A 16-bit counter is used for the total number of blocks. This is sufficient for a single ProDOS-8 volume, but not enough for multiple volumes.

[inexorabletash]

Bleah. Fixing will require:

• a 24-bit counter and all of the operations that check sizes (e.g. copy) will need to handle it
  • Only Download checks the size, so it's not so bad.
• a 24-bit int-to-string conversion

[inexorabletash]

Note that if #749 is done then this may become moot.