Support for Sessions

[SimLV]

Now it is possible to keep multiple sessions and to switch between them.

[SimLV]

@dail8859 I am not sure how to fix that MacOS build problem

[dail8859]

Please undo changes to submodules to keep the changes to a minimum.

Also remove TODO.md. These normally just end up being false promises to users.

[dail8859]

I think the macos-latest was updated and is not able to compile Qt 5.15 as easily. MacOS + Qt 5.15 will probably be dropped unless someone can get the GH action working again.

[SimLV]

Done

[dail8859]

@SimLV Definitely appreciate the PR and nice to see people interested in developing the application more.

My schedule is pretty erratic so I'll try to get to this when I get some free time.

I'm not overly familiar with the Notepad++ session support but I get the general idea of it. If I have any questions I'll let you know.

[SimLV]

I am not sure they had any or not. They are just useful. I added them to NotepadQQ but found project a bit abandoned. So I am going to make some improvements here according to Eating your own dog food principle.

[dail8859]

So would you care to describe this new functionality in a few sentences from a user's perspective? E.g. describe a use-case were they would use it, what can they expect from it, etc

That will help me better evaluate the feature and offer any suggestions.

[SimLV]

As a user I may work on different projects.
So I want to have all files related to current project opened.
Then I may decide to switch to another project so all opened files of current project should be saved into this session and closed. While files from second project should be restored.