Leaving the webpage in Editor mode does not remove member from session

[kossebau]

When the Editor is shown and the user clicks on any link (e.g. to configure his/her account settings or simply a browser bookmark), then currently the cursor is not removed and the editing session is not left from the server POV.
While the server will drop the cursor and the member from the session after a while, if the user returns to the session quickly enough he/she will see the own old cursor still in the session, additionally to the new/current one.

Because the design of WebODF explicitely supports the concept of having multiple cursors from the same user in the document, simply reassigning any existing cursor to the same user on session entering is no option.

Instead the webpage with the Editor running has to try to trigger the cursor removal and the session leaving on the page unload as good as it can.

[VicDeo]

@kossebau I added beforeunload handler in d69c02e

it's probably not enough but still related :-)

[VicDeo]

Disconnect request is implemented in 9b0afdd
To fix this issue it should be sent on unload in synchronous mode. (Most likely it won't work properly in all browsers)