Add a note on how to escape from mouse capture mode

Seriously, who is going to figure that out without reading the source code?!?
Centrally define drive counts in pccore.h

Of course, we would still have to manually edit the .ini file declarations to change the actual number of available drives.
Revamp drive menu

Now showing the loaded image file directly inside the menu. In order to facilitate this, the drive menu is now completely generated in code.

Also, moved the disk creator dialog to the Disk menu. Makes a lot more sense there.