Defines the OnMemoryListRetrievedListener.

The method onMemoryListRetrieved(ArrayList<MemoryModel> memoryList) is used to pass a list of memories loaded to an observer
Makes the list of memories to be retrieved from Dropbox only on startup

The list of memories is stored by the MainActivity in order to be reused by any Fragment, this way the access time is much faster since it's not necessary to reopen the Dropbox datastore.
Define the OnNewMemorySavedListener interface

Through the onNewMemorySaved(MemoryModel newMemory) method the observer is notified when a new memory
is created. In this case the MainActivity adds the new memory to the memory list, causing the MemoryListFragment to reload
but not accessing the (slow) Dropbox datastore over and over again.
Change memory list item color to blue for better visibility.