Need MythPopupBox::showOkPopup() replacement

[Nigel]

There are over 80 popup invocations, mostly informing the user of errors. They should be replaced by something that uses libmythui. This is the obvious replacement:

    MythConfirmationDialog *pop;
    MythScreenStack *stk = GetMythMainWindow()->GetStack("popup stack");
    pop = new MythConfirmationDialog(stk, message, false)
    if (pop->Create())
        stk->AddScreen(pop);
    else
        delete pop;

but 80 copies of that is wasteful. Patch contains a simple global function proposal (with some extra error checking)

[Nigel]

(In [18251]) Replace MythPopupBox::showOkPopup() with non-blocking MythUI replacement, ShowOkPopup?(). This will currently only display correctly where the underlying widgets have been converted from Qt exec() style (like in the main menu where EJECT is called). Closes #5653