Add “Load / Save” feature

[iamdey]

1. Description

Add “Load / Save” feature.

This is a clean history branch of the fork from @vidalsasun that can be merged into the official repository. You can see his work on his repository.

I'm not the author of this changes and I didn't make any change in the code (except linting format), the following is the details of his work:

Screenshots

Since we do not have an alphabetical keyboard, I have decided to name the tracks with the format edit_+ddmmyyyyhhMMss.

If you decide to change the file name later, it will load without any problem.

There are 2 new folders on /home/pi/.config/LMN-3:

• Saved: where all tracks are stored
• load_project: the actual project

When you press the save button:
- the load_project xml file is moved to saved folder, if there are a file whith the same name, it is overwriten

When you Add a new track:
- Load_project folder is cleaned
- A new file is created on Load_project folder and the actual project xml file are moved to saved folder, if there are a file whith the same name, it is overwriten
- LMN-3 program restart

When you load a track:
- Load_project folder is cleaned and the track selected is moved to load_project folder
- LMN-3 program restart

When you save track, you can see the actual project name:

2. Fill in checklist by marking [x]

• I've read the CONTRIBUTING.md
• My code is formatted using required linting
• I've run my code locally and checked it works
• My code has unit tests associated with it (if applicable)
• I've run the test suite locally and all tests pass