tray-on-close

If the SystemTray flag is set the messanger now minimises itself to tray
on close.

It is done by additional argument _closable in BrowserWindow
constructor and additional object TRAY_EVENT in it.

Windows marked _closable=True are closed imidiately. Windows marked
_closable=False are minimised to tray (Main window only).
Merge pull request #1 from letalvoj/tray-on-close

tray-on-close