File Browser: Add “Set As..." context menu

Moves options for configuring:

- Playlist Directory
- Recording Directory
- Start Directory

into a single menu.

Necessary prerequisite for giving the
"Playlist Catalogue" context menu item a title that's a
little more action-oriented ("Add to Playlist…”).

Change-Id: I4ee08fc67d5350c38ae1a57cb345c4ed1082d0b5

manual/configure_rockbox/playback_options.tex

@@ -224,10 +224,10 @@ you to configure settings related to audio playback.
   %
 
 \section{\label{ref:ConstrainAutoChange}Constrain Auto-Change}
-  If enabled and you have set \setting{Start File Browser Here} to a directory
+  If enabled and you have set \setting{Start Directory} to a directory
   other than root, \setting{Auto-Change Directory} will be constrained to the
   directory you have chosen and those below it.
-  See \reference{ref:StartFileBrowserHere}.
+  See \reference{ref:StartDirectory}.
   %
 
 \opt{headphone_detection}{