[applets/appmenu] Fix undefined Alt data source access

In Wayland session data sources for modifier keys might be unavailable, in which case the whole source.data.Alt is undefined. Standard JavaScript try-catch didn't help much, because short-circuiting operator && simply returns left-hand-side undefined value which is falsy. It used to spam error messages to a console at a rate proportional to the number of top-level menu items:

file:///usr/share/plasma/plasmoids/org.kde.plasma.appmenu/contents/ui/main.qml:112:17:
Unable to assign [undefined] to bool

Amends 156415f3

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Update: The rest of this branch was cut off into a separate merge requests

I am not so sure about «Combine three FrameSvgItem into one» commit. And I also haven't figured out the visible leading ampersand bug yet: !1017 (comment 373499)