[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
This has to be accessible from the gui, not hidden in settings. The 'all recordings' folder makes complete sense in PVR, since sticking recordings in a folder per day is not uncommon for backends, which may be nice to structure the filesystem, but not when you want to see a list of all recordings of some show. I don't want to go into PVR settings to flip a switch to flatten the list in order to see those.

So refactoring this code to match the other libraries' methods is okay, but don't change functionality please. Either make this flatten switch available directly from the window, or keep an 'all recordings' magic entry that links to ?flatten=true (or whatever the url is)

> Op 6 aug. 2014 om 09:48 heeft da-anda <notifications@github.com> het volgende geschreven:
> 
> In addons/skin.confluence/language/English/strings.po:
> 
> > @@ -613,6 +613,10 @@ msgctxt "#31511"
> >  msgid "Channel Options"
> >  msgstr ""
> >  
> > +msgctxt "#31512"
> > +msgid "Flatten list"
> > +msgstr "Flatten list"
> 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.
> 
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