Move line up/down should support multiple cursors

[Tyriar]

• VSCode Version: Code - Insiders 1.6.0-insider (20c3935, 2016-09-09T05:59:09.913Z)
• OS Version: Linux x64 4.4.0-31-generic, Ubuntu 16.04

Steps to Reproduce:

1. Add multiple cursors across multiple lines
2. Run the move line up command, only the line containing the "primary" cursor is moved and the other cursors are lost

[alexdima]

The reason is that each cursor produces some edits and those edits overlap. This can only work if the move lines actions are more aware of multicursor than looping over the cursors in:

https://github.com/Microsoft/vscode/blob/master/src/vs/editor/contrib/linesOperations/common/linesOperations.ts

abstract class AbstractMoveLinesAction extends EditorAction {

    private down:boolean;

    constructor(down:boolean, opts:IActionOptions) {
        super(opts);
        this.down = down;
    }

    public run(accessor:ServicesAccessor, editor:ICommonCodeEditor): void {

        var commands:ICommand[] = [];
        var selections = editor.getSelections();

        for (var i = 0; i < selections.length; i++) {
            commands.push(new MoveLinesCommand(selections[i], this.down));
        }

        editor.executeCommands(this.id, commands);
    }
}

PR welcome

@alexandrudima Part of the problem is that only one cursor is assumed to be related to an ICommand. So grouping selections into one command won't work that well, as after you do one line move you'll lose all the other cursors. One possible way to go around that is to simply have the one cursor and select the lines involved. That's not ideal though.

Related to where the selections are reconstructed.

Changing the ICommand would mean rewriting code for a bunch of other derived commands. Not sure about the interaction with multiple cursors of the other commands, at least for SortLinesCommand it doesn't sort the lines, it'll only sort if there's a selection that contains two or more lines. Not sure what the desired behavior is for that or if you still want a PR to fix this issue.

Losing cursors example for a single line move:

[blunket]

would love to see this fixed. i use these specific key combos frequently in sublime. surely this bizarre behavior is not intended, kinda bummed to find out it's a huge change.

[blunket]

@alexandrudima Just curious, why was the "help wanted" label removed here?

[brandonzylstra]

I think that #55780 would also help this problem (but they are certainly not mutually exclusive).

[brandonzylstra]

Also, this looks like it may be a duplicate of #6995, but it's also possible that I'm failing to see important differences between them.