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.

[SimLV]

Could you please look at this and #556

[dail8859]

Apologies for the super long delay. I've been quite busy and only been able to get brief windows of time to put into the project. Hopefully my schedule lightens up soon.

[lucasvmx]

@dail8859 what about this PR?