This adds a 'level' query parameter to qute://log and qute://plainlog.
For example, qute://log?level=warning will show an html page containing
log entries with severity warning or greater.
If the query is omitted, the original behavior of qute://log is
preserved.
:messages [level] is a command that opens qute://log?level=<level>.
By default, level defaults to 'error' as an easy way to see missed
error messages.