add more pt_BR translations and some fixes

Remove the "on" prefix. Events are listened to via "on" method, so it's
redundant.

Data is now by default cached in sessionStorage and not localStorage.
Can be configured via the "storage" option.

Also fixed 2 bugs.

1. Instead of checking whether minimized chats toggle is visible, check whether
there ae aleady minimized chats (visuals might be delayed).
2. Don't call restore from removeChat. Chat gets restored earlier.