plugins: Improve usability of iPod keymaps

- Reduce need to press multiple buttons at
the same time to quit a plugin

- Have "Menu" be default way to quit plugins or
to access plugin menu

- Fall back to (Long) "Select" or Long "Menu"
in cases where Menu button isn't available
(e.g. in ImageViewer and many games)

out of scope:
boomshine, lua_scripts, Rockpaint,
Doom, Duke3D, Pacbox, Quake,
Sgt-Puzzles, Wolf3D, XWorld,
Minesweeper, Pixel Painter, Spacerocks

Change-Id: I6d4dc7174695fe4b8ee9cbaccb21bdbfe6af5c48

manual/plugins/starfield.tex

@@ -8,7 +8,8 @@ Starfield simulation (like the classic screensaver).
   \opt{HAVEREMOTEKEYMAP}{& }
     & Increase / decrease number of stars\\
     
-    \PluginUp{} / \PluginDown
+  \nopt{IPOD_4G_PAD,IPOD_3G_PAD}{\PluginUp{} / \PluginDown}
+  \opt{IPOD_4G_PAD,IPOD_3G_PAD}{\ButtonScrollFwd{} / \ButtonScrollBack}
   \opt{HAVEREMOTEKEYMAP}{& }
     & Increase / decrease speed\\
     \opt{lcd_color}{%
@@ -16,7 +17,8 @@ Starfield simulation (like the classic screensaver).
        \opt{HAVEREMOTEKEYMAP}{& }
         & Change colours\\%
     }%
-    \PluginCancel{} or \PluginExit
+  \nopt{IPOD_4G_PAD,IPOD_3G_PAD}{\PluginCancel{} or \PluginExit}
+  \opt{IPOD_4G_PAD,IPOD_3G_PAD}{\ButtonMenu}
   \opt{HAVEREMOTEKEYMAP}{& }
     & Quit\\
 \end{btnmap}