Engine-XMPP: contacts were able to modify local roster through nickname changes

nicknames are now only processed if there is a non-empty display name for the contact in the roster that differs from the jid
Engine-XMPP: regression caused personmodel of personchat not to be updated
Merge pull request #163 from oli-obk/fix/xmpp_nickname_overwriting
Engine-JabbR: fixed missing full stop in message

Thanks goes to Martin Bagge for noticing this.
Frontend-GNOME: fixed invisible engine assistant on OS X and Windows
Merge branch 'stable'
L10N: Updated Czech translation from Transifex
L10N: Updated Danish translation from Transifex
L10N: Updated German translation from Transifex
L10N: Updated Spanish translation from Transifex
L10N: Updated Swedish translation from Transifex
L10N: Updated French translation from Transifex
L10N: Updated Chinese Simp translation from Transifex
L10N: Updated Turkish translation from Transifex
L10N: Updated Finnish translation from Transifex
L10N: Updated Portuguese (Brazil) translation from Transifex
L10N: Initial partial Persian translation from Transifex
L10N: Initial partial Telugu translation from Transifex
Build: added more Transifex managed languages to update-transifex make target
HACKING: do not specify default access modifiers