Message_TaskInitialise fix, and save 50KB ROM space
- Fix Message_TaskInitialise using the wrong value for the slot size (MemoryLimit vs. ApplicationSpaceSize), which can cause task manager to get confused and show the wrong slot size for a task
- Save 50KB of ROM space by converting the ROM icon sprites to use RISC OS 3.5 mode words (and thus smaller mask data) instead of numbered modes