UI files: use notr="true" instead of old comment="KDE::DoNotExtract"
Usage of "KDE::DoNotExtract" predates the notr attribute and still results in uic generating code trying to do a (bogus) lookup for the very string. (notr attribute exist since many years, at least since 4.8 -> https://doc.qt.io/archives/qt-4.8/designer-ui-file-format.html & scripty's extractrc also supports it since 15 years, so can be relied on now without conditions :) )
With notr attribute instead the generated code will only set the given string as-is. While the given strings are just sample text to help when working on the UI file and ideally would not appear at all in the final product, this here at least fixes the also unneeded translation call.
With the notr attribute (which in Qt Designer is reflected by a checkbox on the string in the property editor) the placeholder strings used here might also be reconsiderated, but not in the scope of this MR, which is all about getting rid of usage of outdated "KDE::DoNotExtract".