App store listing: screenshots, links and UI fixes

Add three screenshots (backups, restore, settings) taken on a demo
instance, plus website, repository and the HOWTO as admin documentation
in info.xml.

Fixes found while taking them:
- the "new folder" restore option lost "<date>" (l10n sanitizes it as
  a tag); show the folder name pattern via a placeholder instead
- the restore-point dropdown cut off the time
- passphrase actions stacked vertically in the slot table
- HOWTO: SFTP example used "sftp::password", the auth mechanism is
  "password::password"

docs/HOWTO.md

@@ -32,7 +32,7 @@ Examples:
 
 ```
 occ backup:target:add offsite amazons3 -a amazons3::accesskey -o bucket=<bucket> -o key=<key> -o secret=<secret>
-occ backup:target:add nas sftp -a sftp::password -o host=<host> -o user=<user> -o password=<pw> -o root=home
+occ backup:target:add nas sftp -a password::password -o host=<host> -o user=<user> -o password=<pw> -o root=home
 occ backup:target:add local local -a null::null -o datadir=/mnt/backup
 ```