Qbs: Do not write all kits all the times into Qbs profiles

Do not push all kits into Qbs profiles all the time. That can be really slow and can thus make startup times of Qt Creator really long (>40s for one user).

Unfortunately there is no way to read out QtEnvironment from qbs after it was set on a profile, so it is really hard to decide when the data needs to be refreshed in the Qbs profiles.

The only way forward that I see at this time is to add a good hash to Kits (and everything potentially included in them) so that we can store that hash in the profile. That is way too big a change for Qt Creator 3.2.