Adding delete file key and playlist context menu option

### Discussion

Discussed at https://github.com/mpc-hc/mpc-hc/pull/28#issuecomment-10993660

> people didn't like the delete option. Personally, I see when it can be useful, but it's what most people prefer

* The context menu item name 'delete from disk' clearly communicate the action and can't easily be confused with f.e. remove from playlist
* Delete require confirmation with a familiar dialog (the system delete dialog)
* Delete can be undone
* I relatively often use a similar option in other programs, f.e.
	* qBittorrent: 'delete' → 'also delete the files on the hard disk'

> … just … point … OnNavigateSkipFile to NavigateSkipFile then why … rename it?
> … why … move that code into a seperate function? All that OnNavigateSkipFile does is call NavigateSkipFile. I see no need for the NavigateSkipFile function, it's code should be moved to the OnNavigateSkipFile function.

How can `OnNavigateSkipFile` return `bool`?

>  changing the key to Shift+Delete

There's not a key to change from because

* the Options → Player → Keys item doesn't have a default key
* there's no playlist key connected to 'Delete from disk'

If the suggestion is to add this action to the `CPlayerPlaylistBar` (`CSizingControlBarG`) widget

* the code for that would be valuable because it's not obvious to me where that's done
* it can be mentioned that the widget would also benefit from having a ctrl + a and shift + click (select multiple items) action

> name the context menu item "Delete from Disk"

Fixed