[pvr] deprecate "All recordings" in favor of a "flatten" feature

Currently when the list of recordings contain one or more folders, a magic "* All recordings" folder is created. This involves some not-quite-obvious path wizardry. This replaces that behavior on favor of a "Flatten list" radio button in the side blade (only visible in the Recordings window).

A couple of issues that are yet to be ironed out:

* I haven't yet checked whether the watched icon overlay works as expected, it was buggy before and I have a feeling it hasn't fixed itself

* `CONTROL_BTNFLATTENLIST` is defined as `500` since there seems to be no convention with regards to how the controls are numbered. If you want me to change this I will.

* The second and third commits are up for discussion. The last one actually fixes something but may be moved to a separate PR if that's what you want

@xhaggi @opdenkamp for review
in case it's used in same context, why not. We probably also should move the "flatten" option to the settings where we moved it for the video library (Setting - > PVR -> Recordings -> flatten). It's nothing you'll change a lot I guess.