EditorManager: Change openEditor/activateEditor to not do mode switching

Also introduce a indirection, modes have now types, edit and debug have
the same type, and editors do have a prefered type of modes instead of a
prefered mode.

That fixes the bug that if the prefered qml editing mode was set to
design, then in switching between qml editors would
- in Edit Mode: stay in edit mode
- in Debug Mode: switch to debug mode

src/plugins/coreplugin/editmode.cpp

@@ -114,6 +114,11 @@ QString EditMode::id() const
     return QLatin1String(Constants::MODE_EDIT);
 }
 
+QString EditMode::type() const
+{
+    return QLatin1String(Constants::MODE_EDIT_TYPE);
+}
+
 Context EditMode::context() const
 {
     static Context contexts(Constants::C_EDIT_MODE,