Extra date/time placeholder descriptions, 12 hour <h>/<hh>

Now you can see extra date/time placeholders with descriptions in the configuration dialog with the "Show More" link button.

Moves "AM/PM or am/pm (localized)", "Hour (00–23)" and "Timezone offset (±0000)" to main list.

Moves "Timezone (abbreviation)" to extras since timezone codes are not standardized or unique.

<h> and <hh> are now always 12 hour format so that they can be meaningfully different from <H> and <HH>.

Updated filename test for <h> and <hh>.

CCBUG: https://bugs.kde.org/show_bug.cgi?id=477215

Depends on !309 (merged)