🍒Plasma/5.27: UI: Greatly refactor prompt dialogs

(cherry picked from commit 86175639)

Due to string freeze policy, ellipsis were replaced back with triple dots.

FolderDialog had to be replaced with an equivalent FileDialog, because that's what is available in Qt 5.15 with QtQuick.Dialogs 1.3 module.

PopupFocusReason didn't work in Qt 5, so I replaced it with TabFocusReason. Not semantically correct, but works.