* Fixes#3145
The system tray menu used to have three items (toggle window,
lock database, quit) of which only two had an icon, which
looked strange and unintended. This commit adds an icon for
the "Toggle window" menu item.
* Use a padlock icon for the "Lock DB" tray menu item
Originally, icon `document-encryt.png` was used, however theming seems
to be getting in the way by sometimes displaying a plain "document"
icon instead. Copied the icon files to `database-lock.png` and used
that one for the "lock database" tray menu item instead.
* In Database Settings, use the "security-high" icon for "Security"
Previously, the "document-encrypt" icon was used, which should be
something like a padlock but which, due to theming, somethings
fell back to a generic document icon (page of paper).
The "document-encrypt" icon is no longer used and was removed.
