[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
;) ... should we use the already existing label #20411 -> "Flatten" instead of this new one?