[Qt Virtual Keyboard] Support setting a maximum size in Desktop integration mode

When using Desktop integration, the Qt Virtual Keyboard (DesktopInputPanel) spans the entire screen's width. This makes it far too big on a widescreen display.

Compare the attached screenshots - the native Windows software keyboard feels balanced, while the Qt Virtual Keyboard feels excessively large.

We need a sensible maximum size, and ideally this default maximum can be configured by the developer.