keepassx: disable systray icon

* Merge from Debian unstable. (LP: #1094568) Remaining changes:
- Add check_systray_available.diff
- Disable the tray icon when running on Unity. (LP: #842224)

Kosszepen.

A hozzatartozo issue:
https://bugs.launchpad.net/ubuntu/+source/keepassx/+bug/842224

Kivancsi vagyok, mire jutnak vegul...

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