Add the ability to cancel a running CD rip.
Merge branch 'master' into ripcd-cancel

Conflicts:
	src/ui/ripcd.cpp
Run 'make format'
Add a mutex to protect a variable shared between threads.
Give the mutex lockers a tighter scope.
Initialise cancel_requested_ to false in the constructor's initialisation list.
Merge pull request #4340 from paperbagcorner/ripcd-cancel

Fix the cancel functionality in the CD ripper