[#2369] [GTKUI] Fix bypassing tray password dialog

Created a generic password dialog and moved the unlock code out of
systemtray so any call to window.present will now show the dialog.

Also fixed the appindicator not showing the correct visible status
[GTKUI] Fix quitting bypassing password lock
[GTKUI] Typo causing password dialog to show
[#2315] [GTKUI] Potential fix for lost window on Win32
Prevent private flagged torrents auto-merging trackers

When adding a torrent already in session any new trackers are merged
to the exiting torrent but this is an unwanted feature for private
flagged torrents.