Application is not closed when the main window's X button is clicked, it's hidden instead, it can be displayed
again by clicking in the system tray icon or choosing 'Show' from the context menu.

This functionality is only available if the system tray is available (as reported by Qt) and if
the desktop environment is not Unity, since the system tray replacement it ships with doesn't work.

On Unix there is no problem in saving the pixmap and opening the file immediately after
to read its content. On Windows, however, the file is always half-written so we get
corrupted data. Cache the icon binary representation and use it in get_image instead
of opening and reading the file directly.

Also, remove local icon when an account removes it from XCAP