Combination dir

[oleg68]

I'm going to start working with combination-related issues: #708, #1195, #1196

This PR moves the combinations-related source files to the separate src/grandorgue/combinations dir.
I've tried to make separate directories for MVC, but unfortunally the current interclass calls do not fullfil the MVC approach.
It is just moving and renaming. No behavior should be changed.

[oleg68]

@rousseldenis could you approve this PR,

[rousseldenis]

@oleg68 Thanks for this.

As a recurrent criticism occurs on some forums about code documentation, when such refactoring occurs, shouldn't we include code comments as much as possible ?

This in order to enhance reviewers understanding and further contributors.

[oleg68]

@rousseldenis you are right. Most of existing code is not documented now and I don't understand it. When I make some changes in a piece of code, I must to understand it and I'm trying to put some comments.

But the changes like this are more formal: some classes and variables are renamed and their files are moved, and it introduces a lot of formal changes to all references to the renamed/moved objects. Most of the changes are made automatically, So I cann't comment them now.

I doesn't consider commenting the existing code as a completable task, but it is a good practice to add comment to a code piece changed manually.

[rousseldenis]

Yes, just a gentle reminder to help people contribute more easily.