Offer choice of default browsers

The database or playlist catalogue can now be set
as the browser that is launched when pressing
ACTION_WPS_BROWSE on the WPS, unless another browser
has more recently been opened.

Previously you'd always have to exit the File Browser
first, after the player had been restarted, which is
annoying for users who prefer the database.

The playlist catalogue has become part of the MRU
browser list, so pressing ACTION_WPS_BROWSE after
selecting a track from a playlist in the playlist
catalogue will now take you back there.

Settings menus have been slightly restructured.

- Eliminated "Set WPS Context Plugin" and "Hotkey" menus
  from the General menu

- Added "What's Playing Screen" menu in Settings-General
  with option for setting default browser. The "WPS Hotkey"
  and "WPS Context Plugin" menu options have been moved to
  this menu.

- "File Browser Hotkey" is now part of the File View menu,
  which means it is accessible from the browser's context
  menu as well.

Overview of resulting menu structure in Settings->General:

What's Playing Screen
    Default Browser
    WPS Hotkey
    Set WPS Context Plugin

File View
    (...)
    File Browser Hotkey

Change-Id: Iaa3619a791c20ce3562a1efd2cf90c72933b729a

manual/rockbox_interface/wps.tex

@@ -61,7 +61,7 @@ your WPS (While Playing Screen).
       %
       \ActionWpsBrowse
       \opt{HAVEREMOTEKEYMAP}{& \ActionRCWpsBrowse}
-      & Return to the \setting{File Browser} / \setting{Database}.\\
+      & Return to the \setting{File Browser} / \setting{Database} / \setting{Playlist Catalogue}.\\
       %
       \ActionWpsContext
       \opt{HAVEREMOTEKEYMAP}{& \ActionRCWpsContext}