Adds a new `dateFormatStr` config key which is used when the `dateFormat` "StringEnum" is set to `custom`.
Shows a link to the Qt time formatting documentation above the text field.
Qt doc link and text field are hidden when not set to custom date format.
CCBUG: 340982
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{F6530111}
{F6530110}
Should I add presets like Event Calendar? https://store.kde.org/p/998901/
{F6530126}
I left out the large text blurb that you can use Rich Text formatting, but Digital Clock can now support using `'<font color="#f00">'ddd'</font>' d` to change the color of the date.
{F6530127}