plugins: random folder advance: misc improvements

fixes:
- traverse_dir was called recursively using two
  MAX_PATH local buffers => possible stack overflow
- The import function inserted additional, incorrect,
  entries when the whole line buffer was filled
- "Dirs not found" message overlapped number
  of folders when generating folder list
- Final number of scanned folders wasn't displayed
- Wouldn't stop inserting when maximum number
  of files in playlist was already reached
- Prevent buffer overflow when importing
- Don't write to fd after opening failed
- Use whole buffer with read_line, instead
  of subtracting 1. Remove hard coded sizes
- CRs don't need to be removed in import function
  (already handled by read_line)

features:
- Use insert context for *much* faster insertion
  of large # of folders, and nicer progress display
- Use UI viewport
- Add progress indicator when saving or loading
- Display number of folders in edit list title
- Go back to plugin's main menu from edit list
- Only ask to save changes if list is dirty
- Warn before erasing modified playlist
- Flag successfully created playlist as modified
- Make folder scan wait for dircache
- Shorten menu item names
- Put "Play Shuffled" menu item first
- Remember selection when returning from submenus
- Go to WPS after ACTION_TREE_WPS
- Exit menu when pressing Back
- Perform an initial scan, if no data file exists
  yet, when "Play Shuffled" or "Edit" is selected

Change-Id: I7df76f8fb8387888ce491d8b74b01b481e9997d3

docs/PLUGIN_API

@@ -1874,13 +1874,14 @@ int playlist_get_track_info(struct playlist_info* playlist, int index, struct pl
     \return
     \description
 
-int playlist_insert_directory(struct playlist_info* playlist, const char *dirname, int position, bool queue, bool recurse)
+int playlist_insert_directory(struct playlist_info* playlist, const char *dirname, int position, bool queue, bool recurse, struct playlist_insert_context *context)
     \group playback control
     \param playlist
     \param dirname
     \param position
     \param queue
     \param recurse
+    \param context
     \return
     \description