Get basic behaviour of landing page right

The landing page is shown if QmlDesigner is disabled and the user opens .ui.qml file.

The landing page informs about Qt Design Studio and shows various options. (See https://bugreports.qt.io/browse/QDS-6563)

The landing page contains a checkbox (a):
Always Open with Qt Design Studio. (Whenever a .ui.qml file is opened we launch QDS instead. This requires a .qmlproject file that will also be opened)
This option is disabled by default. If enabled The landing page is not opened anymore.

Even if the automatic switch to design mode is disabled by the checkbox (a), the user can show the landing page by choosing the design mode for a .ui.qml file. The option is already implemented in qmlproject plugin using the notification bars.

The landing page has to check if QDS is installed. The check is already implemented in the qmlproject plugin.
If QDS is not installed the checkbox (a) can not be enabled.
Instead, a link is displayed (https://www.qt.io/design). This can later be refined to open the maintance tool.

If QDS is installed the landing page contains two buttons. One will open the ui.qml file in QDS the other will switch back to the edit mode.

This functionality should be sufficient to replace the info bars.