This change primarly adds a new `Open with...` entry to the explorer context menu. To do this however, I had to make a few other changes:

- Add a new explorer context key for availible editors
- Moved the editor select prompt into a new function called `openEditorWith`
- Use `openEditorWith` for the new `open with` explorer command as well as for the `reopen with` command

This clarifies that the type is an aggregate over inputs and not editor controls.

* IEditor => IEditorPane

* More panes:
* IVisibleEditor => IVisibleEditorPane
* ITextEditor => ITextEditorPane
* ITextDiffEditor => ITextDiffEditorPane

* imports - remove type alias

* More pane renames
* IEditorService#activeControl => activeEditorPane
* IEditorService#visibleControls => visibleEditorPanes

* More editor renames
* activeTextEditorWidget => activeTextEditorControl
* visibleTextEditorWidgets => visibleTextEditorControls

* add a few more readonly to properties
* move IVisibleEditor closer to IEditor
* remove options accessor from IEditor

forward active (nested) editor event from composite code editors, make activeTextEditorWidget return the active composite editor

Previously we would fire a wrong active-editor-change-event when more than one group is opened and the user clicks into an inactive editor of the inactive group because we would first activate the group and then open the editor.