qt: add preference `qt-backdrop-blur`

This setting makes it possible to disable the backdrop blur filter for the main window.

Apart from accessibility reasons, having this setting enabled makes text being rendered in worse quality since Qt's sub-pixel rendering is impaired by translucency, which is a requirement for exposing backdrop effects in the UI layer.

In order to align with the proposed design, this setting is enabled by default.

Close #28694 (closed).