Change filename placeholder format
Use <placeholder> instead of %placeholder.
Also use more descriptive/straight forward placeholders since we can and migrate old placeholders.
With this new format we could add things like <ISODate> or
<ShortDate> as shorthands for common date/time formats.
We would also have an easier time adding support for more placeholders.
I use <key> instead of %key% because sequences with obvious opening
and closing characters are easier to parse for humans and machines. I
chose '<' and '>' instead of other pairs of characters because I don't
need to escape '<' or '>' in regex. I do need to escape them in Rich
Text/HTML/XML, so it's sometimes less convenient than '{' and '}', but
not too bad.
Since strings are frozen for the Gear 24.02 beta, this patch will probably have to wait for a while.