Some Fixes to the UI behaviour

Moved the delete_event signal handling to the ContactListWindow init_signals()
funcions;
Fixed 2 bugs:
1-Now changing the "minimize on tray" option actually change the behaviour
of the app when the user press the minimize button
(no longer forcing the user to kill and relaunch the application)
2-Fixed the window staying hid when minimized with the "minimize on tray"
 option active
Merge branch 'busmath-master'
Merge branch 'master' into av
fixed bug leading to double freeing the audiomanager instance
Changed strings to more neutral
updated pot file, simplified and removed some strings to translate
Merge branch 'zetok-av-naming' into av
updated german translation
Add Polish translation
added pl translation to cmake
Merge branch 'zetok-translation' into av
api update
Merge branch 'master' into av
no more clang testing for now
retrieve system proxy configuration to use with toxcore
Merge branch 'master' into av
added queued offline messages to todo, updated gitignore
enabled preview function for audio and video
Merge branch 'av' of github.com:jhn1291/Venom into av-testing
finished audio preview function