Add preliminary disk swapping interface.

Works with Mednafen's disk swapping model.
Merge branch 'master' into cd-swap
Merge branch 'master' into cd-swap
Log which SwapInterval implementation is used.
Merge branch 'master' into cd-swap
Add replace/add image interfaces.
Clarify that it's possible to remove images from the list.
Add disk control to retroarch-joyconfig.
Merge pull request #150 from Themaister/cd-swap

Disk-swapping interface