UI files: use consistently notr="true" where needed, not old comment="KDE::DoNotExtract"

Usage of "KDE::DoNotExtract" predates the notr attribute (at least Qt 4.8) and these days still results in uic generating code trying to do a (bogus) lookup for the very string.

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.

KDE's scripty tool's extractrc skips empty string, similar to how Qt's uic skips empty strings and treats them as unset property. So second commit drops unneeded usage of comment="KDE::DoNotExtract" with such strings. Has been only used with a subset of such empty strings, so dropping also yields more consistency.