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

apps/plugins/periodic_table.c

@@ -619,7 +619,6 @@ enum plugin_status plugin_start(const void* parameter)
         switch (button)
         {
         case PERIODIC_KEY_SELECT:
-            break;
         case PERIODIC_KEY_MENU:
             return PLUGIN_OK;
             break;