delegates: Some follow-up fixes and improvements

Sort out QML imports

Add import alias even for types within same module (*TitltSubtitle), and clean up unused imports.

Add null guards for contentItem

Whatever API consumers may be up to, we don't wanna our framework explode with errors inside out.

Port from binding on visibility to direct check

Visibility is an implicitly propagated property. External changes to component's visibility as a whole should not affect its internal layout.

Replicate horizontalAlignment workaround from Label

    // Work around Qt bug where left aligned text is not right aligned
    // in RTL mode unless horizontalAlignment is explicitly set.
    // https://bugreports.qt.io/browse/QTBUG-95873
    horizontalAlignment: Text.AlignLeft

Fix up layout in manual test

There was a random Layout.rightMargin on one of the delegates, and neither RowLayout had an explicit spacing set.