Multi diff text editor has no tab input

To be able to reach to the files presented in a multi diff text editor the tab model should provide a corresponding TabInput (see https://github.com/microsoft/vscode/blob/6504145957b330d6dbbfa83ec6b09c039d4a87ae/src/vscode-dts/vscode.d.ts#L17714)

May be a good idea would be to have a general composite tab input. For the multi diff text editor the elements could then be `TabInputTextDiff`
