ContextDrawer: Clean up and port away from removed stuff

The contextualActions are gone from Kirigami.Page, and so should the associated contextualActionsAboutToShow signal.

There were couple of expressions in this components which were checking for applicationWindow in a wrong way.

That lengthy menu.model binding, thanks to modern QML & JS features is now reduced to a one-liner filter expression wrapped in a top-level method (note: not a property, so we don't have to store the list extra time).

Private action-item delegate was ported to required property and a typed Kirigami.Action kAction companion to shorten expressions.

The "go-up-symbolic" icon is gone, because is was pointlessly overlapping with existing item's icon, and I couldn't find any justification for it being there at all.

Test plan: can be tested on a modifier version of tests/actionsMenu.qml file. Just add the following lines to it:

    contextDrawer: Kirigami.ContextDrawer {
        actions: pageStack.currentItem?.actions ?? []
        enabled: true
    }

    pageStack.globalToolBar.style: Kirigami.ApplicationHeaderStyle.Titles