CLI improvements for Qt 5.14+
- Qt 5.14 adds a "--help-all" option, so include it in the autocompletion.
- Qt 5.14 also warns on multiple additions of the same option, which we accidently did (KAboutData and we ourselves called it), so don't do that.
- Also fixes device autocompletion, which I accidently broke.
Should be cherry-pick to the 1.4 branch too.