Tray: Workaround collection (#5179)

Tray: Workaround collection (#5179)

Tray: Workaround collection

* QDBus workaround for Qt 5.5.0 only, there were reports of the tray
  working fine with 5.5.1. #5164
* OWNCLOUD_FORCE_QDBUS_TRAY_WORKAROUND to force the workaround on an off
* OWNCLOUD_TRAY_UPDATE_WHILE_VISIBLE to enable or disable updating of
  the menu while it's visible - disable by default due to problems on OSX and Xubuntu.
* Track the visibility of the tray menu with aboutToShow/aboutToHide
  only on OSX - the aboutToHide signal doesn't trigger reliably on linux
* Refactor such that setupContextMenu is different from updateContextMenu
* Don't use on-demand updating of the tray menu when the qdbus workaround
  is active, instead to occasional (30s) updates of the tray menu.