Adds a new customDateFormat 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
Should I add presets like Event Calendar? https://store.kde.org/p/998901/
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.