Use action texts for buttons of decision/confirmation dialogs

Merged Friedrich W. H. Kossebau requested to merge work/kossebau/avoidyesnobuttondialog into master

@vkrause @mlaurent

Also recommended by KDE HIG:

Buttons should clearly indicate the available options using action verbs (“Delete”, “Rename”, “Close”, “Accept”, etc.) and allow the user to make an informed decision even if they have not read the message text. Never use “Yes” and “No” as button titles.

Things get a bit tricky/not yet mentioned in KDE HIG when it comes to the confirmation dialogs when cancelling editing: the usual "Cancel" button in a dialog about confirming a cancel operation will not work (similar confusion about confirming a "close"), so the button wordings need to be different here. What I have seen elsewhere is that instead of "Cancel" then the negation of the action is declared, like "Do not close" or "Do not cancel", and otherwise not repeating text from the confirming button, to be more distinct. Thus the current proposal.

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