Reworks to the shuffle system to improve performance and allow fast shuffling from a big library (but this work for all database views)

This improvement brings a huge performance improvement to start a random mix of your library. Previously, the only way to do this was to increase the size of a playlist with absurd sizes number. Now it will respect the limitation but will insert random songs from the current view.

Database: Add true random songs in playlist if it is gonna exceed its maximum capacity

More context is available here : https://www.reddit.com/r/rockbox/comments/1ez0mq4/i_developped_true_full_library_shuffle_for/

Also :
- Improved layout in the DB browser
- New default max playlists capacity is now 2000 on old PortalPlayer targets to give a better user experience and not having to wait dozens of seconds while creating a playlist
- "Show insert shuffled" option is now true by default
- Add a new shortcut to play all songs shuffled in the DB browser
- Now the feature is fully optional and enabled only on targets that have more than 2MB of RAM
- Add entries about this feature in the manual to explain it to the users
Change-Id: I1aebaf7ebcff2bf907080f1861027d530619097c

Change-Id: I3354923b148eeef1975171990e814a1a505d1df0

apps/gui/wps.c

@@ -806,7 +806,7 @@ long gui_wps_show(void)
                     theme_enabled = false;
                     gwps_leave_wps(theme_enabled);
                     onplay(state->id3->path,
-                           FILE_ATTR_AUDIO, CONTEXT_WPS, hotkey);
+                           FILE_ATTR_AUDIO, CONTEXT_WPS, hotkey, ONPLAY_NO_CUSTOMACTION);
                     if (!audio_status())
                     {
                         /* re-enable theme since we're returning to SBS */
@@ -823,7 +823,7 @@ long gui_wps_show(void)
             {
                 gwps_leave_wps(true);
                 int retval = onplay(state->id3->path,
-                       FILE_ATTR_AUDIO, CONTEXT_WPS, hotkey);
+                       FILE_ATTR_AUDIO, CONTEXT_WPS, hotkey, ONPLAY_NO_CUSTOMACTION);
                 /* if music is stopped in the context menu we want to exit the wps */
                 if (retval == ONPLAY_MAINMENU
                     || !audio_status())