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/projectexplorer/foldernavigationwidget.cpp

@@ -267,7 +267,7 @@ void FolderNavigationWidget::openItem(const QModelIndex &srcIndex)
     }
     // Open file.
     Core::EditorManager *editorManager = Core::EditorManager::instance();
-    editorManager->openEditor(m_fileSystemModel->filePath(srcIndex));
+    editorManager->openEditor(m_fileSystemModel->filePath(srcIndex), QString(), Core::EditorManager::ModeSwitch);
 }
 
 void FolderNavigationWidget::setCurrentTitle(QString dirName, const QString &fullPath)