Fix crash when changing session without DBus

Review Request #127238 - Created March 1, 2016 and submitted - Latest diff uploaded

Kåre Särs
cullmann, jowenn
When changing session Kate trys to check if the session is open in another instance, but if DBus is missing we get a crash. This patch checks the DBus sessionBus pointer and pretends that there are no other instances running.

Do we need to add some lock-file mechanism to ensure we don't have two instances modifying the same session or just add a dialog warning the user if DBus is missing?
Compiled Kate on windows and am now able to change session without getting a crash.